Thanks again for your reply. My Mam is still having problems with her email.
She has Avast Business Security, as shown in the screenshot in the attachment the "Mail Shield" option is greyed out so it is permanently on and I cannot turn it off but the scanning of outgoing mail is disabled. Also, as shown in another screenshot in the attachment, the Windows Defender firewall has been disabled, I cannot find a firewall with Avast Business. Despite these I am still having problems.
I have managed to link my own Gmail account to my Mam's Outlook and I managed to send and receive emails no problem, so there is obviously no problem with the outlook, wifi connection it is something to doe with the Virginmedia email account. It did work with no issues until last week when it stopped working despite making no changes to the account settings.
Is is your mam running a business that she requires Avast’s business product? Is it possible she or you could uninstall the product to run it out as the problem? Have you checked for any Avast updates? Have you restarted Avast? Have you restarted the PC?