First off, this seems to be a snippet of the error in the log. Including it all could help me get closer.
Verify:
That the email server configured exists on the hostname and port.
That you can ping it from JIRA
That you see the port open while doing a nmap from the JIRA server
Are you using a local sendmail or an external mailserver?
Talk to the mailserver admin:
Does the mailserver support the protocol you chosen? (IMAP/IMAPS)
The fact that you see authentication error makes me think you gave it credentials to send email, but in most intranets this is not needed.
Look at things that might have changed since it last worked.
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