Have done as you suggested using all variants of sending I can do from here:
1)Via Freeserve dial up Pop account
2)Via Virgin dial up Pop account.
3)Via Oneandone dial up Pop account.
4)Via Virgin broadband to Pop account.
5)Via Virgin broadband to Oneandone Pop account.
...and nothing has been bounced yet! It's going to be one of those situations where the problem has mysteriously been sorted, and we'll never know for sure why it happened.
Did try sending to Freeserve Pop via Virgin broadband but got this:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'firstname.lastname@example.org'. Subject 'Testing7', Account: 'Freeserve', Server: 'smtp.freeserve.com', Protocol: SMTP, Server Response: '550 Administrative prohibition', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79