Why Your Transactional Email Lands in Spam
Key takeaway
Authenticate your domain with SPF, DKIM, and DMARC, send transactional mail from a different subdomain than marketing, and monitor bounces and complaints. Most deliverability problems are one of those three, and a password reset that doesn't arrive is a support and security problem, not a marketing one.
Transactional email — password resets, receipts, notifications — is infrastructure your product depends on, and it's usually configured once by someone who has since left. Then a support queue fills with "I never got the email".
Authenticate the domain properly
- SPF — a DNS record listing who may send for your domain. Watch the lookup limit; too many included services silently breaks it.
- DKIM — cryptographically signs your mail so recipients can verify it wasn't altered or forged.
- DMARC — tells receivers what to do when SPF and DKIM fail, and sends you reports. Start at none to observe, then move to quarantine and reject.
- Set up all three. Missing authentication is the most common reason legitimate mail is filtered.
Separate your sending streams
Send marketing campaigns and transactional mail from different subdomains. Reputation is tracked per sending domain, so a campaign that generates complaints should not be able to stop password resets from arriving. Use a dedicated IP only if your volume is high enough to sustain a reputation; at low volume, a good provider's shared pool is better.
Reputation is behavioural
- Hard bounces must suppress the address immediately — repeatedly mailing dead addresses damages reputation quickly.
- Honour unsubscribes instantly, and include a working list-unsubscribe header on anything marketing-adjacent.
- Never send to purchased or scraped lists; spam-trap hits are close to unrecoverable.
- Warm up a new domain or IP gradually rather than sending a large volume on day one.
- Monitor complaint rates and act on the trend, not on individual reports.
Content and structure
- A real reply-to address on a monitored inbox; no-reply addresses depress engagement signals.
- Plain text alongside HTML, and a sensible text-to-image ratio.
- Avoid link shorteners and mismatched link domains — both are strong spam signals.
- Keep transactional mail transactional. Adding promotional content changes its legal classification in several jurisdictions and its filtering treatment everywhere.
Monitor it like a service
Track delivery, bounce, and complaint rates, and alert on deviation. Test the critical flows — signup, password reset, receipt — against the major mailbox providers after any DNS or provider change. Deliverability degrades quietly, and the first person to notice is usually a customer who can't get into their account.