Why Some Submissions Come Back “Unverifiable” (and What To Do)

Turn on SMTP verification and you’ll soon see a status that surprises people: unverifiable. It’s not a failure — it’s an honest "we couldn’t get a definitive yes or no." Understanding why keeps you from blocking real customers.

The common causes

• Blocked outbound port 25. If your host blocks port 25, Spamify can’t reach mail servers at all. The port-25 probe will tell you if this is the cause.
• Greylisting. Many servers deliberately give a temporary "try again later" to unknown senders. That’s a non-answer, not a rejection.
• Catch-all domains. Some domains accept mail for any address, so the server can’t say a specific mailbox doesn’t exist.
• Timeouts. A slow server that doesn’t answer within your smtp_timeout window.

The key setting: smtp_unverifiable_action

Because unverifiable is ambiguous, Spamify lets you decide its fate with smtp_unverifiable_action:

• Flag for review (default, recommended) — the submission goes through but is marked so you can eyeball it. No real customer is ever turned away.
• Block — only sensible if you’ve confirmed clean port-25 connectivity and prefer to err strict.

For most sites, flag-for-review is the right call: you still get syntax validation, honeypot, timing, and rate limiting doing their jobs, and the genuinely-dead addresses are caught by the hard_block_smtp rule regardless. Watch your dashboard for a week and only tighten if you see abuse slipping through the flagged bucket.