What Is the Warm Up Process?
The warm up process is the gradual method of building a positive sending reputation for your domain and IP addresses.
When you begin sending from new IPs (or a new platform), mailbox providers do not yet trust your sending behavior. Instead of allowing high volumes immediately, they evaluate your reputation over time.
Warm up works by:
Sending small volumes first
Mailing your most engaged subscribers
Gradually increasing volume as performance remains strong
As inbox providers observe positive engagement (opens, clicks, low complaints), they begin allowing more of your mail to reach the inbox.
If you previously mailed successfully from this same domain at another ESP, the warm up period may be shorter but it is still required.
Why Is IP and Domain Warm Up Necessary?
It may seem easier to send your full database immediately. However, mailbox providers do not yet recognize your new sending environment.
Without a warm up:
Mail may be throttled (temporarily limited)
Messages may land in spam
Your IP reputation may be damaged early
Modern spam filtering systems evaluate both:
Domain reputation
IP reputation
Whether you are using shared IPs or dedicated IPs, building a reputation gradually is essential. In today’s email landscape, there is no safe shortcut.
Think of it as building trust. Inbox providers reward consistent, positive behavior over time.
How to Properly Warm Up
1. Start With Your Most Engaged Subscribers
Your first sends should go to subscribers who are most likely to engage:
Recent openers
Recent clickers
Recent replies
These subscribers help establish positive engagement signals.
If migrating from another ESP:
Segment subscribers based on recent activity.
Prioritize the most active contacts first.
If starting from scratch:
Your list growth will naturally act as a warm up.
A double optin process is strongly recommended.
2. Use Permission-Based Lists Only
Do not send to:
Purchased lists
Scraped emails
Contacts who did not explicitly opt in
Sending to unverified or non-consented contacts can permanently damage your reputation during warm up.
3. Prepare Your Technical Setup
Before sending, ensure:
SPF is properly configured and verified
DKIM is properly configured and verified
Tracking links and image URLs use your sending domain
Your domain authentication is fully aligned
Authentication builds trust and improves deliverability.
4. Clean Your List (If Migrating)
If moving from another ESP:
Use a reputable list hygiene service
Remove inactive or invalid addresses
Eliminate old hard bounces
5. Segment and Control Volume
Segment by major mailbox providers (Gmail, Outlook, Yahoo, etc.)
Break larger lists into smaller send groups. This is easy using the virtual segments.
Increase daily volume gradually
Warm up is not one-size-fits-all. Warm up speed depends entirely on:
Engagement rates
Complaint rates
Bounce rates
Inbox placement performance
If engagement is strong, volume can increase more quickly.
If engagement drops, slow down and optimize content before increasing volume.
What to Monitor During Warm Up
Throughout the process, monitor:
Open rates
Click rates
Bounce rates
Spam complaints
Your spam complaint rate should remain below 0.1%.
If performance declines:
Improve subject lines
Refine targeting
Strengthen content relevance
Slow down volume increases
When Is Warm Up Complete?
Warm up is considered successful when:
You can send at your intended volume
Engagement remains stable
Complaint rates remain low
Inbox placement is consistent
Even after warm up is complete, reputation management remains ongoing.
Maintaining list hygiene and strong engagement is essential for long-term success.