The situation you've described is a common problem with both Outlook and Outlook Express, so I wouldn't be at all surprised if the cause (and the solution) are the same when using eM:
Email clients seem to have a habit of screwing up the data relating to the outgoing server (SMTP) settings. That can happen even when users examine the settings which are shown within the program and find them appearing to be OK.
When you tried an 'on top' installation of eM the program retained the corrupt data, so that didn't fix the problem.
The solution which has worked every time I've encountered this problem (both on my own machines and when responding to questions on AB) is to completely delete the relevant email account within the client and then to create it again. (I'd advise restarting your computer in between those two steps but I'm not actually sure that it's absolutely necessary). Deleting the account won't, of course, delete any existing emails.
If you're using a Virgin Media email address you can check the relevant server settings here:
https://help.virginmedia.com/system/templates/selfservice/vm/help/customer/locale/en-GB/portal/200300000001000/article/HELP-2203/Email-Settings-for-all-Virgin-Media-domains