The mail error is solved since 08:13 today (2019-05-23). The rejected email(s) have to be resent.
We currently have a problem with certain mail sent to us is rejected.
The error message that the sender receive is:
"Client was not authenticated to send anonymous mail during MAIL FROM> # SMTP # and Could not deliver to the following recipients or groups: [email address] @ lnu.se <mailto: [email address] @ lnu.se> Your message was not delivered because the recipient's email provider rejected it. "
Ask the sender to resend the letter.