It's a long time since I have seen one of these. The error VM321 means that too many authentication attempts have been seen in every hour. VM will deny further attempts. VM apply these limits to prevent spam but do disclose what the hourly limits are. because to do would subvert the point of the limits. My suspicions are that these attempts may not be yours. If a malicious actor has knowledge of your email address (easy) and your password (not so easy but entirely possible) they could be sending spam mail which would trigger this error. You wouldn't see this but it it will be clocking up authentication attempts on your address and account.
So the first thing to do is to change your password in My Virginmedia. You will then need to wait 24 hours for the counters to reset and for you to be able to send mail again.
Of course this could be down to you, I don't know. You might be sending lots of email because you are working from home. Remember this is a residential mail service and business use is limited to some home working. You might want to create some fall-back capacity with an address from one of the free (ad-supported) services.
While the Forum Team can intervene it won't be of much help immediately. They will simply raise an IT ticket with a minimum 5 days resolution time.
I think the immediate solution lies in your hands. But I'm not ruling out a VM issue. It's just that there are things you should be doing first which might resolve it quickly.
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