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

Using Windows 10

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:

"

An Unknown error has occured

Server: 'smtp.blueyonder.co.uk'

Windows Live Mail Error ID: 0x8007007A

Protocol: SMTP

Port: 465

Secure (SSL): Yes

"

We tried telling it to "Repair all Windows Essentials" Programs, allowed the very first Test email to be sent, but then the problem reoccurred for all subsequent emails.

Uninstalling and reinstalling WLM did not solve the problem

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