Database mail trying to send 2 times and then failing

Our database mail was running for 3 years without problems, but since one week, we are constantly getting this error:

The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 1 (2021-10-13T22:31:21). Exception Message: Cannot send mails to mail server. (Failure sending mail.). )

Unfortunately this message is not very helpful. Database mail is always trying to send the mail 2 times (with 1 minute interval), then it finally fails.

  • Is there a way to find out why exactly it is failing? ?
  • Is there a way to reconfigure database mail, so that it tries more often or sends in other ways?