I am being plagued with client errors trying to send through my SMTP server (postfix).
On a fast internet connection it works no problem, no error. On a slow (satellite) connection, it constantly returns this error:
Task 'firstname.lastname@example.org - Sending' reported error (0x800CCC60) : 'Your outgoing (SMTP) email server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP). The server responded: c220 box.tuilder.com ESMTP Hi, I'm a Mail-in-a-Box (Ubuntu/Postfix; see https://mailinabox.email/)'
This is an outlook client, but mac mail clients have the same problem.
It seems that there is a timeout occurring somewhere along the line. Not sure if I can tweak a postfix setting. Have tried a few, but no progress.