Deliverability warmup

Deliverability

The gradual increase in email volume from a new domain or inbox to build sender reputation before full campaign deployment.

What is Deliverability warmup?

Deliverability warmup is the process of gradually building a positive email sending reputation for a new domain or inbox by starting with low-volume, high-engagement sending and incrementally increasing volume over 4 to 8 weeks before sending at full campaign scale. Email providers judge new senders as higher-risk until they demonstrate a track record of sending wanted emails with high engagement and low complaints.

Warmup tools automate this process by sending emails between their own networks of seed inboxes, ensuring those emails are opened, replied to, and marked as not-spam, which builds positive reputation signals with inbox providers before your actual campaign outreach begins. Popular warmup tools include Mailreach, Lemwarm, and Instantly's built-in warmup feature.

The warmup period is not just about technical reputation. It also establishes sending patterns that providers recognise as normal for your domain. A domain that has been sending 10 emails per day for 30 days and then suddenly sends 500 will trigger pattern-break alerts even if the underlying reputation is clean. Gradual, consistent volume increases are less likely to trigger spam filtering than sudden jumps.

Warmup should never stop entirely during active campaigns. Even during live outreach, maintaining some warmup sending in the background preserves and rebuilds reputation between campaign waves, especially when bounce rates or spam complaints have been elevated. Treat warmup as ongoing maintenance rather than a one-time setup step.

This matters because deliverability is cumulative. Small issues in authentication, volume, or list quality can quietly erode inbox placement over time, even when individual campaigns still look acceptable on the surface. It usually becomes more useful when it is defined alongside Domain warmup, Inbox placement, and Domain reputation.

Deliverability warmup — example

A new sending domain is set up for an outbound campaign. Warmup starts with 10 emails per day in week 1, increases to 20 in week 2, 40 in week 3, and 80 in week 4. During warmup, inbox placement tests show 95% landing in primary inbox by week 3. At week 5, campaign outreach begins at 30 emails per day, staying below the maximum established during warmup. Open rates in the first campaign are 38%, indicating clean inbox placement. Without warmup, similar campaigns from cold domains consistently produce open rates below 15%.

A B2B company rolling out cold email formalizes Deliverability warmup as part of setup rather than as a rescue tactic. They define who owns the checks, what metrics are reviewed weekly, and which actions are off-limits during warmup. They also make sure it connects cleanly to Domain warmup and Inbox placement so the definition is not trapped inside one team.

Frequently asked questions

How long does email warmup take?

A minimum of 3 to 4 weeks before any cold outreach begins. 6 to 8 weeks before scaling to high volumes. The more conservative your warmup, the more protected your domain reputation. Rushing warmup to launch campaigns faster risks short-term gain with significant long-term deliverability damage.

Do I need to warm up every new inbox separately?

Yes. Each domain and inbox needs its own warmup. If you set up five inboxes across two domains for a campaign, each inbox should go through the warmup process independently. Warmup at the domain level affects the domain's overall reputation. Warmup at the inbox level affects that specific address's reputation within the domain.

Can I warm up and run cold campaigns simultaneously?

It is generally recommended to complete warmup before starting cold outreach on a domain. Running cold outreach during warmup risks elevated bounce rates and spam complaints that will damage the reputation you are trying to build. If you need to launch quickly, use a pre-warmed domain from your existing infrastructure while the new domain completes warmup.

How do I know when warmup is complete and I can start sending campaigns?

Run an inbox placement test using GlockApps or a similar tool. If 90% or more of test sends land in the primary inbox across major providers, the domain is ready for campaign sending. Also check that your domain has no blacklist entries using MXToolbox. Start campaigns at a daily volume below your warmup peak rather than immediately jumping to maximum capacity.

What happens if I skip the warmup process entirely?

Your emails are significantly more likely to land in spam, particularly with Gmail and Microsoft 365 which aggressively filter unrecognised senders. Bounce rates may also be higher if the domain has no sending history. Skipping warmup typically produces open rates 50% to 70% below what a properly warmed domain would achieve on the same campaign, negating whatever time was saved.