Emails sent to me are being bounced back

Last week I was waiting for several email responses from companies, eventually decided they were just not replying.  However on Sunday I had a text from Tesco Bank telling me that my email address was undeliverable and I would have to log into my account as normal to be able to view the message they were trying to send me.  Then I remembered 2 weeks ago a friend had exactly the same issues, so we sorted it through Messenger.

I tried to speak to Virgin about it, and they seemed to think it was because my account wasn't verified - I log into My Virgin Media through a virgin email address, even though to access this email account I do log in with another email!  Getting my drift?

Before they'll help further they want me to change the MVM login.  I tried and it failed - no email being sent to my other non-virgin email address.  They've opened a ticket on that bit, but meanwhile I'm stuch not kow what emails I'm missing.

Today I've got my friend to try and email me again - once again it bounced back with the error message: 

Server error: '550 <*****@virginmedia.com.> invalid address '*****@virginmedia.com.''

But this is correct, and has been for the last 20 years, and it was working for her up until about 10 days ago.

Can anyone help with this please.