AFAIK the error code VM321 occurs when too many unsuccessful authentication events have occurred within an hours, i.e. SMTP server is no longer attempting to validate presented authentication credentials and simply blocking access. Make sure no further attempts to authenticate with the SMTP server occur for an hours, then re-run your troubleshooting steps for Outlook and post the resulting VM code here.
Are you able to sign into webmail using the same authentication credential that failed?
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I re ran the logging as suggested after about an hour and authentication was this time successful and email sent. This is the same behaviour as I have regularly seen over the last few weeks - locked out for a period then authentication works - only to be blocked again.
The logging result was: <rx> 235 2.7.0 SMTP Authentication successful for user ....
However, this evening we are back to to having authentication failed with the same result as before:
<rx> 525 5.7.13 Authentication Denied (VM321)
I can confirm webmail is working normally. I also confirm that my laptop is fully virus protected and recently fully scanned for bots etc. It is also behind an enterprise firewall on the network.
Forum team are best placed to investigate this issue further; issue has been flagged to them however be aware it can take them a few hours / days to respond. In the meantime review any devices from which the affected email account is/was checked in case one or more have incorrect authentication password thereby triggering the intermittent authentication issue.
Thanks for your post and apologies to hear you're having an issue with your emails through Outlook.
The error code VM321, indicates to many emails being sent each hour. As you've mentioned that Webmail is working, I would suggest you remove all your settings from Outlook and try re-adding them with the help from this link: virg.in/mailset
Give it a try and let us know how it goes. If things still don't work, we may need to escalate this to Internet Security.
I have already tried resetting all the outlook settings following the details provided. Unfortunately can you escalate this to Internet security. I am assuming that their may be a third party continually trying to send email via my account (unsuccessfully I understand but it still lock me out).