Out of the blue, since this morning, whenever I send an email from the domain dooms.eu (served by mailcow) I receive an error message: Your message did not reach some or all of the intended recipients. Subject: FW: Test Sent: 6/26/2023 3:24 PM if(window.hljsLoader && !document.currentScript.parentNode.hasAttribute('data-s9e-livepreview-onupdate')) { window.hljsLoader.highlightBlocks(document.currentScript.parentNode); } The following recipient(s) cannot be reached: Luc Dooms | Agio Capital on 6/26/2023 3:24 PM Server error: '451 4.3.0 luc@dooms.eu: Temporary lookup failure' if(window.hljsLoader && !document.currentScript.parentNode.hasAttribute('data-s9e-livepreview-onupdate')) { window.hljsLoader.highlightBlocks(document.currentScript.parentNode); } Restarting SOGo does not help. What can I do?

Server error: '451 4.3.0'

Out of the blue, since this morning, whenever I send an email from the domain dooms.eu (served by mailcow) I receive an error message:

Your message did not reach some or all of the intended recipients.

  Subject:  FW: Test
  Sent:  6/26/2023 3:24 PM

The following recipient(s) cannot be reached:

  Luc Dooms | Agio Capital on 6/26/2023 3:24 PM
        Server error: '451 4.3.0 luc@dooms.eu: Temporary lookup failure'

Restarting SOGo does not help.

What can I do?

You could start finding out what SMTP error 451 actually means.
Have you any rate limits configured?

Thank you, I opened a case at Mailcow. They helped me, lessons learned. Do not manage Mailcow with sudo; use root access instead.