Please empty your outbox first!
Check whether the connection is configured for NTLM and not password. If so, try switching the authentication to password.
Please make sure the OS and email client software are up2date.
Test whether the password is correct, log in to Webmail with all your accounts that are used in the network and set the correct passwords everywhere. Do not copy passwords from Office applications (word docs etc.) copy ONLY from a plain text editor.
SMTP settings:
Please tick advanced
SMTP server needs authentification
Authentication mechanisms = plaintext (PLAIN, LOGIN)
Restart your computer and devices afterwards.
The server response was:
I had this issue with iOS after recent update where the server (postfix based) refused incoming email from the client.
As a bypass to solve it I did this: Changed protocol for authentication of outgoing (SMTP) email for the account from « NTLM » to « Password » on the iPhone and the fake smtpclient.apple domain is no longer sent in the HELO and the server is happy.
Details:
https://discussions.apple.com/thread/252756298