Bulk Email Delivery Issues: Connection Timeouts to External SMTP Servers

Help with Setup/Upgrade
1

Hi Mail-in-a-Box Community,

I need help with bulk email delivery from my Mail-in-a-Box server. I’m running version 3.6.4 on Ubuntu 22.04 with Postfix 3.6.4. All DNS records including MX, SPF, DKIM, and DMARC are properly configured.

We’re trying to send around 100 emails to Gmail and Yahoo addresses. While the first few emails deliver successfully, subsequent attempts result in connection timeouts. The messages get stuck in the queue with errors indicating connection timeouts to the external SMTP servers.

I’ve verified that port 25 is open and working through telnet tests. Direct SMTP connection tests to both Gmail and Yahoo servers are successful. The firewall is correctly configured to allow port 25 traffic, and sending individual emails works without issues.

To troubleshoot, I’ve tried implementing various solutions like adding delays between sends, reducing batch sizes to 3-5 emails, separating the sending queue by domain, and adding retry logic. I’ve also tried clearing the mail queue between attempts. Despite these efforts, the connection timeout issues persist.

The error logs consistently show messages like “status=deferred (delivery temporarily suspended: connect to mail-server:25: Connection timed out)”. This happens after successfully delivering a few initial emails.

I’m seeking guidance on Mail-in-a-Box’s recommended settings for bulk mail delivery, suggested rate limits for different email providers, and best practices for handling mass mailings. Also wondering if there are any known issues with bulk sending in the current version.

Any help or direction would be greatly appreciated.

Thanks!

2

No expert here, but I recall a few forum posts saying that MIAB isn’t that suitable for bulk / mass emailing and generally recommend tools made for the purpose instead.

From personal experience, I can tell you that Google, Yahoo, Microsoft etc. do not even like if MIAB is used for personal use only. They are quick to block. And even after you through their un-block procedures, they will block you again, if they feel like it. There are ways around it with anydomain.net (for example, no affiliation).

3

@Mg344
I can confirm that if you see deffered then they will escalate you to a block. Try registering yourself as bulk sender in Postamster tools and following their unsuscribe button guidelines. Otherwise send via relay.

 To reduce the amount of spam sent to
    Gmail, this 550-5.7.1 message has been blocked. For more information, go to
    550 5.7.1  https://support.google.com/mail/?p=UnsolicitedMessageError

They just don’t like self hosted email servers.