1. On a desktop: running Thunderbird 32-bit. Clicking "Get all new messages" works properly every time. Never fails.
2. On a laptop: also running Thunderbird 32-bit. Clicking "Get all new messages" sometimes works, but often fails with "Error with account email@example.com -- Sending of username did not succeed. Mail server pop3.virginmedia.com responded: Invalid Backend.".
From that post: ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Inbound: Server: pop3.virginmedia.com Port: 995 Security: SSL Username: Your full email address e.g. firstname.lastname@example.org ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Using this example, I had only put "joebloggs", not "joebloggs@blueyonder .co.uk" as the Username.
However, my Username has actually been set up to just "joebloggs" for a long, long time, but only now started failing.
The answer is not immediately obvious but easy to explain what you saw. If you use the the legacy servers then you are not required to enter the full e-mail address as your username. If you use the virginmedia.com servers then you are. T/Birds automatic set up should do this without your invention but if you set up manually you might forget this step.
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I've been manually configuring email since around 1920-ish (or was it 1820-ish ...?), mainly because back in those halcyon days there was no other way to do it. I've now learnt my lesson, and I now know what's special about virginmedia.com.