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Cold email deliverability checklist: 20 checks covering domain authentication, warmup, list quality, and monitoring. Run this before every campaign launch.
Sarah Okonkwo
Sales ops specialist, deliverability obsessive · Updated June 24, 2026
Last updated: June 2026 · Sarah Okonkwo, sales ops specialist, deliverability obsessive
TL;DR — 5 things to know before reading
Deliverability is the part of cold email that most teams underinvest in until something breaks. The pattern is consistent: a team spends significant effort on copy, buys a solid contact list, builds a thoughtful sequence — and then sends from a domain that has been live for three weeks with no warmup and no authentication configured. Open rates come back at 4%. They assume the copy is wrong. The copy is not wrong. The emails are landing in spam.
The 20 checks in this guide are the checklist I run through before every campaign launch. They are not advanced configurations; they are baseline requirements that determine whether your emails reach inboxes at all. Run this checklist before your first send. Run it again before every new campaign. The ten minutes it takes is the most valuable ten minutes in any cold email setup.
Domain authentication is the technical foundation that tells receiving mail servers your emails are legitimate. Without it, major providers treat your outbound as suspicious regardless of content.
Sender Policy Framework (SPF) is a DNS record that specifies which mail servers are authorized to send email on behalf of your domain. When your SPF record includes Inframail’s sending infrastructure, receiving mail servers can verify that your emails are coming from an authorized source.
What to check: Look up your domain in MXToolbox’s SPF checker (search for “SPF check” in their toolbox). The record should exist, should not have syntax errors, and should include the IP ranges or hostnames for your sending infrastructure. An SPF record with more than 10 DNS lookups will fail the lookup limit and is treated as invalid.
Common failure: SPF record published but not including the actual sending server IP. If you are sending via Inframail, the SPF setup is configured automatically as part of onboarding — confirm the record is live before sending.
DomainKeys Identified Mail (DKIM) adds a cryptographic signature to each outbound email. Receiving servers use this signature to verify the email has not been modified in transit and comes from an authorized sender.
What to check: Use MXToolbox’s DKIM checker or an email testing tool such as mail-tester.com. Send a test email and verify the DKIM signature is present and passes verification. Look for “DKIM: pass” in the authentication results.
Common failure: DKIM configured for the root domain but not the subdomain being used for sending, or DKIM record added to DNS but not yet propagated (propagation takes up to 48 hours).
DMARC (Domain-based Message Authentication, Reporting and Conformance) builds on SPF and DKIM to specify what receiving servers should do with emails that fail authentication. A DMARC policy at minimum p=none allows monitoring without rejection; p=quarantine or p=reject provides stronger protection for your domain reputation.
What to check: Look up your domain in MXToolbox’s DMARC checker. The record should exist. For cold email sending domains, p=none is an acceptable starting configuration — it enables monitoring without risk of accidentally rejecting legitimate email during setup.
What to avoid: No DMARC record at all. Google and Yahoo authentication requirements specify that bulk senders must have a DMARC record published. Sending at volume from a domain without DMARC risks deliverability penalties at Gmail and Yahoo.
When Instantly tracks link clicks and email opens, the default tracking domain is a shared domain used by many senders. A custom tracking domain (a subdomain of your own sending domain) separates your tracking reputation from other users of the same platform. A blacklisted shared tracking domain can affect deliverability for all senders on it.
What to check: In Instantly, confirm a custom tracking subdomain (e.g., track.yourdomain.com) is configured and active for each sending domain. This requires a CNAME record added to your domain’s DNS.
Newly registered domains have lower trust scores with receiving mail servers because spammers frequently register new domains to rotate around blacklists. Cold email from domains less than 30 days old faces significantly higher spam filter rates at Gmail and Outlook even with perfect authentication.
What to check: If you are setting up a new sending domain, register it at least 30 days before your planned campaign start date. 60 days is better. Use Inframail’s domain setup workflow to configure authentication on the new domain immediately upon registration.
Cold email that cannot receive replies loses valuable response data and signals to some receiving systems that the domain is not a functioning email domain. Configure an MX record so replies are delivered to a monitored inbox.
What to check: Confirm an MX record exists for your sending domain using MXToolbox’s MX lookup. If you are using Inframail, MX records are configured as part of the domain setup process.
Email warmup is the process of gradually increasing sending volume from a new domain or inbox over time, building sending history that receiving mail servers use to establish trust. Skipping warmup is the single most common cause of deliverability failure.
Four weeks is the minimum warmup period for a new sending domain or inbox. The Woodpecker warmup and recovery guide recommends starting at 5 to 10 emails per day and increasing by 5 to 10 per day each week. After four weeks of consistent warmup, a domain or inbox is typically ready to support cold email volume at 30 to 50 emails per inbox per day.
What to check: Confirm the warmup start date for each sending domain and inbox. Domains and inboxes started less than four weeks ago are not ready for cold campaigns.
Manual warmup (sending emails back and forth between domains you control) is unreliable because it lacks the network diversity that receiving servers look for. Warmup tools automate sending between a large network of real inboxes, creating the diverse sending history that builds genuine domain reputation.
What to check: Confirm warmup is active in Instantly for every sending inbox. Do not pause warmup during a live cold email campaign — warmup and campaign sending can run simultaneously, and pausing warmup during an active campaign can cause reputation to decay.
According to Woodpecker’s analysis of daily send limits, the recommended ceiling for cold email is 30 to 50 emails per inbox per day after warmup is complete. Above this threshold, spam filter rates increase and inbox reputation degrades more quickly.
What to check: In your sequence settings in Instantly, confirm the per-inbox daily limit is set at or below 50. If you need higher volume, add more inboxes rather than increasing the per-inbox limit. Inframail provisions multiple Microsoft 365 inboxes at scale with automatic DNS configuration, which makes inbox expansion straightforward.
Sending 500 emails at exactly 9:00:00 AM is a spam signal. Human email sending has natural variation — emails trickle out over hours, not in batches. Sequence tools that randomize send time across a window (e.g., 8:00 AM to 5:00 PM with random intervals) produce send patterns that look more like human behavior.
What to check: In Instantly, confirm the sending schedule uses time randomization rather than a fixed send time. Set business hours that match the recipient’s time zone where possible.
Inbox rotation distributes send volume across multiple inboxes within the same campaign, reducing the per-inbox sending load and spreading campaign volume across multiple sender reputations. Three inboxes rotating across a 150-email/day campaign each send 50 emails — within the safe threshold.
What to check: Confirm inbox rotation is configured in Instantly for campaigns above 50 emails per day. For high-volume campaigns, set up Inframail domains and inboxes in batches to build a rotation pool.
Warmup tools sometimes pause automatically if the inbox has not been active or if the account has connectivity issues. A paused warmup causes domain reputation to decay during active sending.
What to check: Log into Instantly and verify warmup status shows “active” for every inbox in your rotation. Check that warmup has been sending successfully for the past 7 days by reviewing the warmup activity log.
List quality determines bounce rate, which is one of the most heavily weighted signals receiving mail servers use to evaluate sender reputation. A bounce rate above 3% is a threshold that causes deliverability problems at most major providers.
Unverified contact lists produce bounce rates that damage domain reputation quickly. Verification checks whether an email address exists at the domain before a send is attempted. Quarvio delivers contacts pre-verified for email deliverability, which means bounce rates stay well below the 3% threshold that triggers reputation damage at receiving mail servers.
What to check: Confirm your contact list source verifies addresses before delivery. If you are using a list that is not pre-verified, run it through an email verification service before importing to your campaign.
The 3% bounce rate threshold is cited by Google’s email sender guidelines as a point at which sender reputation suffers. Consistent bounce rates above this level cause Gmail to route email to spam and eventually to reject delivery.
What to check: In Instantly, review bounce rate metrics for previous campaigns using the same sending domains. If prior campaigns show bounce rates above 3%, investigate the list source before sending the next campaign. Do not add unverified contacts to a domain with an existing bounce rate problem — the damage compounds.
Role-based email addresses (info@, support@, admin@, hello@, sales@, contact@) are not individual professional contacts. They route to team inboxes, are frequently unmonitored, and are often used by spam filters as indicators of bulk sending. Including them in cold outreach lists increases bounce rates and spam complaint rates.
What to check: Review your contact list import for role-based addresses and remove them before upload. Quarvio lists target individual professional contacts by title, which excludes role-based addresses by default.
Sending the same email sequence to the same address twice is both a deliverability problem and an experience problem. Receiving mail servers notice patterns of duplicate sends and treat them as bulk sending indicators. Prospects who receive a sequence twice are more likely to report as spam.
What to check: De-duplicate your contact list by email address before importing into your campaign tool. Most CRM tools and spreadsheet applications have built-in de-duplication. Instantly also has duplicate detection at import.
Where your contact data comes from matters for deliverability and compliance. B2B contacts at companies where the individual has no reasonable expectation of professional outreach generate higher spam complaint rates. Contacts sourced from professional databases focused on B2B decision-maker roles produce lower complaint rates because the outreach matches the professional context of the address.
What to check: Confirm your contact source is a B2B professional contact database, not a compiled list from unknown sources. Review your CAN-SPAM compliance obligations and confirm your opt-out handling is in place.
Active monitoring catches deliverability problems before they compound. Most deliverability failures that take months to recover from could have been caught and corrected within days if monitoring was in place.
Blacklisting occurs when a domain is added to a blocklist maintained by organizations that track spam sources. Major receiving providers (Gmail, Outlook, Yahoo) check incoming email against these blocklists. Blacklisted domains see delivery rates drop dramatically — sometimes to zero at certain providers.
What to check: Run your sending domains through MXToolbox Blacklist Check weekly. This tool checks against 100+ blocklists and shows which ones have flagged your domain. If your domain appears on a blocklist, submit a removal request through the relevant blocklist’s process. Minor blocklists can be removed within days; major blocklists (Spamhaus, Barracuda) take longer and require demonstrating that the spam source has been addressed.
Google Postmaster Tools provides sender reputation data directly from Gmail’s perspective. The dashboard shows domain reputation (High, Medium, Low, Bad), IP reputation, spam rate, and authentication pass rates for email sent to Gmail.
What to check: Register your sending domains in Google Postmaster Tools. Check domain reputation weekly during active campaigns. A reputation drop from High to Medium warrants investigation; a drop to Low requires immediate campaign pause and deliverability review. A Bad reputation means Gmail is actively routing your email to spam.
Google’s email sender guidelines state that spam complaint rates above 0.1% begin to cause deliverability issues and rates above 0.3% cause significant problems at Gmail. Spam complaints are generated when recipients click the “Report spam” button in Gmail.
What to check: Monitor spam complaint rate in Google Postmaster Tools and in Instantly’s campaign metrics. Complaint rates above 0.1% indicate the list quality or outreach relevance needs to be reviewed. The most common cause is sending to contacts who are not a genuine fit for the ICP, which increases the likelihood that recipients report rather than reply.
| Need | Tool | Notes |
|---|---|---|
| Verified, low-bounce contacts | Quarvio | Pre-verified, title-filtered B2B lists |
| Email inboxes with auto DNS | Inframail | Microsoft 365, SPF/DKIM/DMARC auto-configured |
| Warmup, rotation, monitoring | Instantly | Warmup network, inbox rotation, spam rate tracking |
| LinkedIn outreach | Aimfox | Connection campaigns alongside email sequences |
How long does it take to recover from a blacklisted sending domain?
Recovery time depends on which blacklist flagged the domain and how many spam complaints were generated. Minor blocklists can be resolved in two to five business days by submitting a removal request and demonstrating corrective action. Major blocklists (Spamhaus SBL, Barracuda BRBL) can take two to four weeks and require demonstrating that the sending behavior that caused the blacklisting has stopped. Spamhaus and similar lists require submitting a removal request with documentation of the corrective steps. During recovery, all sending from the affected domain should pause. The practical outcome is that a blacklisted domain is often more economical to retire than to recover — which is why prevention is the only viable strategy.
Can I run warmup and cold email sequences simultaneously?
Yes, and you should. Instantly is designed for simultaneous warmup and campaign sending. The warmup network continues building domain reputation while campaigns run, which is especially important in the first six months of a new sending domain’s life. Pausing warmup to run campaigns is a common mistake that causes reputation to decay during peak sending periods. Keep warmup active continuously.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent delivery failure: the address does not exist, the domain does not exist, or the receiving server has permanently rejected the sender. Hard bounces should be removed from your list immediately and never retried. A soft bounce is a temporary failure: the mailbox is full, the server is temporarily unavailable, or the message is too large. Soft bounces can be retried after a delay, but repeated soft bounces from the same address become effective hard bounces. Most sending tools distinguish between the two and handle retry logic automatically. The 3% bounce threshold applies to hard bounces specifically.
How many sending domains should I set up for a cold email campaign?
One inbox per 30 to 50 emails per day means a 300-email/day campaign needs six to ten inboxes. Inboxes should be distributed across multiple domains rather than concentrated on one domain — three inboxes per domain with two to three domains is safer than six inboxes on one domain. If one domain gets blacklisted, spreading across multiple domains means you lose one-third of your capacity rather than all of it. Inframail provisions Microsoft 365 inboxes with automatic DNS configuration, which makes setting up multiple domains and inboxes significantly faster than manual configuration.
Deliverability starts before your first send
Inframail handles domain authentication automatically. Instantly runs warmup continuously. Start with a verified contact list from Quarvio and your bounce rate stays where it needs to be.
