@Beth_G
Please see my posts a couple of ones earlier than the one to which you replied in order to get an idea of the type of "responses" I myself have been getting in the very (and linked in my post!!!!) similar issue that I am currently experiencing with my own "XYZ@virginmedia.com" a/c (via Outlook 2016 & 2019 on different m/cs, and ALSO on my Android phone!) - i.e. I am continually prompted for my P/W because, somehow, the VM outgoing email server (smtp.virginmedia.com) won't accept it.
As I also said in my own thread, a similar thing happened a few weeks ago, and thus I STRONGLY suspect that the outgoing mail server username & P/W database is not up-to-date, or has been corrupted, in the last day or so. Thus, as I also said in my own thread "Here we go around the mullbery bush, AGAIN"! :(.
In the current situation, I have had to remove that email a/c from Outlook on both m/cs to avoid being continually prompted for my P/W (it's correct because I can get into that a/c via webmail and then see all the latest emails - both incoming and outgoing, of which for the latter, of course, there have been none since around midday on Tuesday because the "system" won't let me send any 😡!