Verifiable Provider
business
Self-Hosted (Postfix / Exim / Sendmail) Email Verification
Self-hosted email servers running Postfix, Exim, or Sendmail are used by businesses, universities, and organizations that manage their own mail infrastructure.
How Self-Hosted (Postfix / Exim / Sendmail) Handles Verification
Self-hosted mail servers are typically the most reliable for SMTP verification. Standard configurations of Postfix, Exim, and Sendmail reject unknown recipients with 550 and accept known ones with 250.
However, behavior varies significantly depending on configuration:
- Postfix with
reject_unverified_recipient— reliable verification - Postfix with catch-all virtual aliases — accepts everything
- Exim default — typically rejects unknown users
- Greylisting enabled — first attempt rejected with 450, retry succeeds
What Mailthentic does
- Identifies as "custom_smtp" when MX doesn't match known providers
- Full SMTP verification with greylisting retry (up to 3 attempts with 30/60/120s delays)
- Catch-all detection via random address probes
- Circuit breaker for servers returning 421/452 (overloaded/rate limiting)
Quick Facts
MX Pattern
Varies (custom hostnames)
Catch-All Behavior
Rejects unknown addresses
Typical SMTP Response
Usually 550 for invalid, 250 for valid (configurable)
Provider Type
business
Best Practices for Self-Hosted (Postfix / Exim / Sendmail)
- Self-hosted servers give the best verification results. SMTP responses are typically accurate.
- Handle greylisting. First attempt may be rejected — retry after 30+ seconds.
- Check for catch-all configuration. Some admins enable it unknowingly.
- Verify SPF/DKIM/DMARC. Self-hosted servers often have misconfigured or missing authentication records.
- Respect connection limits. Small servers may only handle a few concurrent SMTP connections.
Other Email Providers
Verify Self-Hosted (Postfix / Exim / Sendmail) email addresses
Our 9-point verification engine handles Self-Hosted (Postfix / Exim / Sendmail)'s specific behavior automatically. Start free.