What server logs are critical to pull during a deliverability audit?

You're digging into why your emails aren't landing where they should. Server logs are your evidence. But not all logs matter equally. here's what to actually pull.

SMTP transaction logs show the conversation between your server and the recipient's. Look for connection attempts, server response codes (that 250 means success; a 421 means the recipient's server is temporarily rejecting you), and timing information. These tell you whether the handshake worked.

Bounce logs are where the rejection details live. Each bounce has a code and message attached. A 550 is a permanent hard bounce (usually means invalid address or authentication failed). A 421 or 450 is a soft bounce (temporary issue, try again later). Capture the timestamp, recipient, and what context was being sent. This data reveals patterns: are you hitting spam traps, or is it real list quality issues?

Authentication logs tell you what happened when your mail server tried to sign with DKIM, verify SPF, or present DMARC. Did the signature get created? Did the SPF lookup work? Any failures here are red flags to validate your setup immediately.

Connection logs capture TLS negotiation details, IP addresses used, and connection timeouts or failures. If you're getting refused at the TLS stage, that's a trust issue. often auth-related.

ESP-specific logs depend on your platform. SendGrid, AWS SES, and Postmark all surface event webhooks, delivery status notifications, and engagement events differently. Pull what your platform offers. Once you've gathered these, look for patterns: are failures concentrated on one mailbox provider? One IP? One time of day? That's where diagnosis lives. If you're stuck interpreting what you find, that's where a Review My Emails SOS call can accelerate the fix.