I think Virgin Media has created their own blacklist to protect their outbound servers.
The Forum Team should be able to assist here, but it can take them a while to get to the thread.
However I would point out that your server settings are out of date as well. Not to mention you are using software that has not received any security updates since 2016.
Consider changing your email client. There are many out there, but Thunderbird remains free and easy to set up. Primarily because Virgin Media have set up autoconfig information for you.
Tim
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