You sign up for an ESP, get a dedicated IP, and send your first 10,000 emails. Half land in spam. Nothing is broken. Your DNS is correct. Your content is clean. The problem is simpler than that: nobody knows your IP yet.
ISPs — Gmail, Outlook, Yahoo — assign reputation to sending IPs. A new IP has no reputation. No reputation doesn't mean neutral. It means untrusted. And untrusted means spam folder.
What warm-up actually is
Warm-up is the process of gradually increasing sending volume on a new IP so ISPs can build a reputation profile for it. You start small, send to your most engaged recipients, and slowly ramp up over several weeks.
The logic is straightforward. If you send 50 emails on day one and every recipient opens them, ISPs record positive signals — opens, clicks, no bounces, no spam complaints. The next day you send 100. Same positive signals. Over time, ISPs accumulate enough data to trust the IP with higher volume.
The standard schedule looks like this:
| Week | Daily volume |
|---|---|
| 1 | 50-100 |
| 2 | 200-500 |
| 3 | 500-1,000 |
| 4 | 1,000-5,000 |
| 5 | 5,000-10,000 |
| 6+ | 10,000+ |
That's a minimum of 6 weeks before you can send at full volume. During this entire period, you need to be selective about who you send to. Your most engaged contacts — people who have opened or clicked in the last 30 days — go first. Cold or unengaged contacts come last, after the IP has established trust.
Why warm-up fails
Most warm-up failures come from the same handful of mistakes:
Sending to unengaged recipients during warmup. If your first 100 emails go to people who haven't opened an email from you in six months, expect low opens, no clicks, and possibly spam complaints. ISPs see this and flag the IP immediately.
Inconsistent volume. Sending 200 emails on Monday, nothing on Tuesday through Thursday, then 500 on Friday. ISPs want to see steady, predictable patterns. Gaps look suspicious. Each gap partially resets the trust you've built.
Stale lists. If your contact list has a high bounce rate — dead addresses, typos, abandoned inboxes — those bounces poison your IP's reputation during the most fragile period. Clean your list before you start warmup, not after.
Spam complaints from cold audiences. If recipients don't recognize your sender name or didn't expect your email, they hit "report spam." Even a 0.3% complaint rate during warmup can set you back weeks.
How Mailgun handles it
Mailgun has the most detailed warm-up documentation in the industry. They treat warm-up as a first-class concern: dedicated IP pools, detailed ramp-up guides, and tooling to manage multiple IPs at different stages of warmup.
Their recommendation is clear: if you send more than 50,000 emails per month, get a dedicated IP and warm it up properly. Below that, use their shared pool. They give you the tools and the documentation, but the execution is on you. You manage the schedule. You segment your engaged contacts. You monitor the metrics and adjust.
This works well for teams with deliverability expertise. It's a lot of operational overhead for teams without it.
How Postmark handles it
Postmark takes the opposite approach: no dedicated IPs at all. Every sender uses shared IPs. Their bet is that strict sender vetting — aggressive enforcement of their terms of service — keeps the shared pool clean.
If you violate their sending policies, you're removed from the platform. No warnings, no second IP to fall back on. This means their shared IPs maintain high reputation because bad senders get kicked off before they can cause damage.
The result: no warm-up needed. You sign up, verify your domain, and start sending. The shared IPs already have established reputation. The tradeoff is that Postmark only allows transactional email. No marketing, no campaigns, no bulk sends. That's how they keep the pool clean.
How PushMail handles it
We use a layered approach that gives most senders zero-warmup onboarding while still supporting dedicated IPs for high-volume senders.
Default: managed shared IPs with subuser isolation. Every PushMail organization gets its own SendGrid subuser. You share the IP pool with other PushMail senders, but your reputation is tracked independently. If another sender has issues, it doesn't affect your deliverability metrics. No warm-up required. You can start sending on day one.
Add-on: dedicated IPs with automated warm-up. For senders doing 100,000+ emails per month, we offer dedicated IPs at $40/month. We handle the warm-up automation — gradually ramping your volume over 6 weeks, prioritizing your most engaged contacts, and monitoring bounce and complaint rates. You don't manage the schedule manually.
BYOK: bring your own key. If you already have a SendGrid account with warmed IPs, connect it to PushMail and keep your existing IP reputation. You get PushMail's sequences, campaigns, and contact management running on infrastructure you've already invested in warming up.
When to skip warm-up entirely
You don't need to warm up an IP if you're not using a new dedicated IP. Specifically:
Use shared IPs. PushMail's default. The IPs are already warmed with established reputation. Subuser isolation protects you from other senders.
Use BYOK with an already-warmed account. If your SendGrid account has been sending consistently for months, those IPs are warm. Connecting PushMail doesn't change your IP reputation.
Send fewer than 50,000 emails per month. At this volume, a dedicated IP doesn't have enough consistent traffic to maintain reputation anyway. Shared IPs will outperform a dedicated IP that only sends a few hundred emails a day.
The bottom line
Warm-up is a real constraint, but it's only relevant if you're provisioning new dedicated IPs. Most senders never need to think about it. Shared IPs with proper isolation give you established reputation from day one.
If you do need a dedicated IP — because you're sending at scale and want full isolation — plan for 6 weeks of ramp-up and only send to engaged recipients during that period. Or let us automate it.