A new sending domain has no history with receiving mail servers. ISPs treat email from unknown domains with automatic suspicion — meaning even legitimate, well-designed emails land in spam folders until the domain earns trust. Domain warm-up is the process of building that trust gradually by demonstrating consistent, clean sending behaviour before ramping to full production volume.
Why Domain Warm-Up Is Essential
ISPs maintain reputation scores for every domain they see email from. A domain that has:
- Never sent email before (zero history)
- Not sent in more than 6 months (dormant)
- Had its DNS records recently changed (suspicious activity)
...is treated as high-risk. Without warm-up, sending 10,000 emails from such a domain on day one will result in the majority landing in spam — even if the content is perfectly legitimate and the recipients consented.
Warm-Up Prerequisites
Before starting warm-up, ensure all of these are complete:
| ✓ | Requirement |
|---|---|
| □ | Domain added and verified in MigoSMTP with SPF, DKIM, and DMARC records passing |
| □ | Domain registered with Google Postmaster Tools for reputation visibility |
| □ | Contact list is clean — all recipients are genuine opt-ins; no purchased lists |
| □ | Email content is fully tested; unsubscribe link verified working |
| □ | Bounce and complaint webhooks are configured to monitor delivery in real time |
Week-by-Week Domain Warm-Up Schedule
| Period | Max Daily Volume | Who to Send To | Key Metric Targets |
|---|---|---|---|
| Days 1–3 | 50 | Internal team, known contacts, seed accounts — people guaranteed to open and not mark as spam | Open rate >50%; zero complaints; zero hard bounces |
| Days 4–7 | 200 | Most engaged recent subscribers — opened in last 15 days; highly motivated audience | Open rate >40%; bounce <1%; complaint rate 0% |
| Week 2 | 500 | Engaged subscribers — opened in last 30 days | Open rate >30%; bounce <1.5%; complaint <0.05% |
| Week 3 | 1,000 | Active subscribers — opened in last 60 days | Open rate >25%; bounce <2%; complaint <0.1% |
| Week 4 | 2,000 | All subscribers who have opened at least once in last 90 days | Delivery rate >95%; bounce <2%; complaint <0.1% |
| Month 2 | Up to plan daily limit | Full list — including less-engaged subscribers | Delivery rate >97%; maintain all metrics in healthy range |
| Month 3+ | Full production volume | All subscribed addresses including dormant list reactivation | Full production metrics maintained |
Metrics That Trigger Automatic Warm-Up Pause
Stop your warm-up immediately and investigate if any of these thresholds are crossed during the warm-up period:
| Metric | Pause Threshold | Likely Cause |
|---|---|---|
| Bounce rate | > 3% on any single send | List quality — invalid addresses; purchased list; old data |
| Spam complaint rate | > 0.1% | Content or consent issue — recipients did not expect your email |
| Delivery rate | < 90% for a single ISP | ISP has flagged your domain — check postmaster tools for the specific ISP |
| Open rate | < 15% during weeks 1–2 | Email may be landing in spam — run inbox placement test immediately |
Domain Warm-Up for a Dormant Domain
If you have an existing domain that has not sent email in more than 6 months, treat it exactly like a new domain — start the full warm-up schedule from day one. Dormant domains lose their established reputation and are treated with similar caution as brand-new domains by most ISPs.
Monitoring Warm-Up Progress
- Google Postmaster Tools (postmaster.google.com) — check your domain reputation rating daily during warm-up. It should progress from Low to Medium to High over the warm-up period.
- MigoSMTP Delivery Reports — monitor delivery rate, bounce rate, and complaint rate after every send during warm-up.
- Inbox placement testing — use mail-tester.com or GlockApps to periodically test whether your emails are landing in the inbox or spam folder.