I have a postfix email server that handles mail via two routes. First is a relay on port 25, this is dedicated to a single external IP address via a 'mynetworks' entry. This route is to handle email from a second server which hosts dovecot and roundcube. For technical reasons I cannot host the postfix server on the webmail server.

Roundcube therefore just sends emails to port 25 on the SMTP server (which resolves via RDNS to an in-domain address) and these are currently being sent-on successfully to gmail email addresses. I have checked via telnet from other hosts that the server is not acting as an open relay and it is not.

However, I want external users to be able to send email from smartphones and standalone email apps, and they could be behind any IP address. Therefore, I've also enabled TLS on procmail with SASL to allow authenticated users to send mail via the SMTP server. Users are successfully authenticating and the server is accepting emails from them. But they are not sending - the procmail queue reports the following error:

host ALT2.ASPMX.L.GOOGLE.COM[74.125.68.26] said: 421-4.7.0 [XXX.XXX.XXX.XXX 15] Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited. Please visit 421-4.7.0 https://support.google.com/mail/answer/81126 to review our Bulk Email 421 4.7.0 Senders Guidelines. xw4si1747676pac.34 - gsmtp (in reply to end of DATA command))

How can gmail be happily accepting emails sent via one method while rejecting emails sent via another from the same postfix server?

I'm very confused. Any ideas would be welcomed.

Sounds like you are authenticating on port 25, this assumes you are connecting as another server and thus originating emails from those device's IP Addresses.

Re-configure your endpoints to use SMTPS Submission port 587.

25 is for server to server.

587 is for client to server.

You can also anonymise those client connections

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