Production SMTP

Operating SMTP servers at scale:

Reputation:

• IP reputation (sender's outbound IP).
• Domain reputation (From: domain).
• Built over weeks/months. Burned in seconds.
• New IPs need "warming up" — slowly increase volume.

Anti-abuse:

• Rate limit per sender, per domain, per IP.
• Greylisting: temporarily reject (4xx) on first contact; legit senders retry, spammers don't.
• Blocklists (Spamhaus, SpamCop, Spam Eating Monkey): consult on inbound.
• DNSWL: allowlists.

Spam classification:

• SpamAssassin: rule-based + Bayesian.
• ML models: feature extraction from headers + body.
• User feedback: Mark as Spam → train classifier.

Deliverability monitoring:

• DMARC aggregate reports.
• Postmaster tools (Gmail, Outlook).
• Bounce rate, spam folder rate.
• Active monitoring: send to seed accounts, track outcomes.

Bounces & feedback loops:

• Proper DSN parsing.
• Hard bounces (5xx): remove address from list immediately.
• Soft bounces (4xx): retry; remove after persistent failure.
• FBL (Feedback Loops): Yahoo, AOL forward "Mark as Spam" to senders.

Compliance:

• CAN-SPAM (US): unsubscribe link, physical address, no false headers.
• CASL (Canada): explicit consent.
• GDPR (EU): consent + right to unsubscribe + data minimization.
• List management: every bulk email needs a List-Unsubscribe header.

Self-hosting realities:

• Major providers (Gmail, Microsoft) suspicious of small senders.
• Need: PTR records, reverse DNS, SPF, DKIM, DMARC, established IP, low bounce rate.
• One missed step → straight to spam folder.

Service providers:

• SendGrid, Mailgun, Postmark, Amazon SES: high-deliverability transactional mail.
• Pre-warmed IP pools, dedicated IPs for big senders, automatic bounce management.
• API simpler than SMTP.

Don't:

• Roll your own outbound MTA without expertise.
• Send bulk mail without warming.
• Buy email lists.

Do:

• Use a service for transactional + bulk.
• Self-host for receiving + relaying internal mail.