SMTP timeout errors when sending large batches

#40 Resolved

Amanda P. 25 days ago

Campaigns to lists over about 500 contacts are timing out partway through with SMTP timeout errors, but smaller campaigns go out fine. Using our own Postfix relay.

Ryan Taylor Staff 25 days ago

A self-hosted relay like Postfix can definitely hit this if it's not configured for sustained connections/throughput — a few things to check: does your Postfix instance have any per-connection rate limiting or max message count per connection configured? Also, is the SMTP connection timeout on the SubsiMail side (Settings > Sending > Advanced) set higher than your network's default, in case the relay is just slow under load rather than actually failing?

Amanda P. 24 days ago

Checked Postfix config — we do have smtpd_client_connection_rate_limit set fairly low from when we set this up years ago for spam prevention. That's probably it.

Ryan Taylor Staff 23 days ago

That would absolutely do it — a low per-client rate limit on your own relay will throttle SubsiMail's connection just like it would any other client. Since SubsiMail is a trusted internal sender in this case, it's usually fine to raise or exempt that limit specifically for SubsiMail's server IP in your Postfix config, rather than removing the protection entirely.

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