Running cold email campaigns for multiple clients is a deliverability minefield. One bad list burns a domain. One domain’s bounce spike cascades through a shared inbox pool. This guide covers agency-specific deliverability infrastructure: per-client domain strategy, isolated sending, warmup protocols, list verification SOPs, and how to diagnose and recover from deliverability drops.
The single biggest deliverability risk for agencies is shared sending infrastructure. If Client A’s bad list burns a shared IP or inbox pool, Clients B, C, and D suffer too.
Never run cold outreach from a client’s primary domain. Register a dedicated secondary domain for each client’s cold email campaigns.
Every list must go through verification before it enters the sequencer. This is non-negotiable at agency scale - your reputation is an aggregate of every campaign you run.
| Step | Action | Threshold / outcome |
|---|---|---|
| 1 | Deduplicate the raw list | Remove duplicate email addresses before verifying - don’t pay to verify the same address twice |
| 2 | Run through BounceZero | Upload CSV > get result, score, is_disposable, mx_valid per address |
| 3 | Suppress invalid | Remove all result: “invalid” addresses - these will hard-bounce |
| 4 | Suppress disposable | Remove all is_disposable: true - fake trial accounts, not real prospects |
| 5 | Triage catch-all | Score ≥ 0.6: include. Score < 0.4: suppress. Keep middle band in “to review” segment |
| 6 | Check final ratio | If >30% are invalid before suppression, the list source is low-quality - flag to client |
| 7 | Load clean list | Only the verified-clean list enters the sequencer. Never skip step. |
Stricter - new domains have no positive reputation buffer
Standard threshold - act before Google flags
More buffer, but never ignore - recovery takes weeks
Register a secondary domain per client separate from their primary (clientco.com instead of client.io), warm it for 3-4 weeks, cap daily volume at 50-100 per inbox, verify every list with BounceZero before loading, and monitor bounce rates daily. Pause any domain above 3% hard bounce immediately.
Isolated per client. Shared infrastructure means one client’s bad list can burn the IP or inbox pool for all clients. Use Smartlead sub-accounts or separate Google Workspace accounts per client so one deliverability incident cannot cascade.
Hard bounce below 2% per sending domain is the target. Pause above 3%. For new domains (< 30 days warmed), treat any rate above 1% as a signal to pause. Above 5% triggers a Google Postmaster Tools reputation drop that can take weeks to recover.
BounceZero is built for agency list hygiene at scale. Bulk upload, per-domain scoring, catch-all triage. $3/1K. Team accounts with shared credits available.
Provider behaviour, DNS setup, warm-up, and inbox placement
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