Windows Live Mail 2012 will not send emails on VirginMedia email account !!!

I believe that VirginMedia email accounts are hosted by Gmail.

WLM receives email on this account OK.

WLM has worked perfectly with this account for the last four years, and then suddenly this problem appears.

We tried telling it to "Repair all Windows Essentials" Programs, which fixed the problem for four hours, but now the problem has reoccurred.

I'd be most grateful for suggestions on how to fix this problem please.