Operating Since 2015
An ESP migration is two projects running in parallel: the technical work of moving templates, automations, and subscribers; and the deliverability work of warming new IPs while gracefully decommissioning the old reputation. Most migration failures happen because teams treat these as a single project and rush the second one. The phases below separate them cleanly.
Plan for a 10-12 week elapsed window even if the technical work compresses to 2-3 weeks. IP warming is the binding constraint: it cannot be accelerated without damaging reputation on the new infrastructure, and the parallel-running of old and new ESPs during warming is what makes the migration safe.
Phase 1: Pre-Migration (Weeks -4 to -1)
The pre-work that determines whether the migration runs smoothly or stops mid-cutover. The subscriber export with engagement data is the single most-important artifact — without it, warming has to start from scratch instead of targeting your most-engaged segments first.
- Choose sending subdomain (mail.yourdomain.com — not primary domain)
- Export subscriber list: email, name, subscription date, last open date, last click date
- Export suppression list (unsubscribed, bounced, complained)
- Document all active automations/autoresponders for rebuild
- Export all email templates
- Confirm infrastructure provisioning timeline (3–5 business days)
Phase 2: Infrastructure Setup (Week 0)
One week of intensive setup work. The DNS changes propagate over hours so plan the work in the right sequence: SPF first (additive change, safe), then DKIM (new selector, also additive), then DMARC last (the policy that depends on both). Postmaster Tools and SNDS need 24-72 hours to begin populating data after first sends, so register them early.
- DNS records added: SPF (new IPs), DKIM (new selector), DMARC (p=none), PTR/rDNS
- DNS propagation verified (all records resolving correctly)
- Test email authentication verified (dkim=pass, spf=pass in headers)
- Google Postmaster Tools: sending domain verified
- Microsoft SNDS: new IPs registered
- Subscriber import with engagement data mapped
- Suppression list imported
- Email templates imported/recreated
- Tracking domain + SSL configured
Phase 3: IP Warming (Weeks 1–10)
The schedule below assumes a conservative warming pace. Aggressive variants reach full volume in 6-7 weeks; very conservative ones stretch to 12-14 weeks. The right pace depends on the engagement quality of your most-engaged segment (the first warming cohort) — if 30-day openers show 35%+ open rate during week 1, you can accelerate; if they show under 25%, slow down.
| Week | Segment | % of List |
|---|---|---|
| 1–2 | 30-day openers only | 5–15% |
| 3–4 | 60-day openers | 15–30% |
| 5–6 | 90-day openers | 30–50% |
| 7–8 | 180-day openers | 50–75% |
| 9–10 | Full active list | 75–100% |
Phase 4: Traffic Cutover
The cutover is gradual, not a switch. Maintain the old ESP at decreasing volume even after the new infrastructure reaches full warming — reputation accumulates faster when traffic patterns stay predictable, and the 30 day buffer at 100% on new infrastructure proves stability before the old reputation is abandoned.
- Postmaster Tools shows High domain reputation for 2+ consecutive weeks
- SNDS shows Green for all new IPs for 5+ days
- Deferral rate below 5% across all ISPs
- Gradually migrate traffic: 50% → 75% → 100%
- Cancel old ESP subscription after 30 days at 100% on new infrastructure