Next time you email one of these contacts that incorrectly prefixes your email address with Virgin consider Bcc yourself on the email (press ⌥ + ⌘ + B to show Bcc field) and then:
- select the sent email when received
- press ⌥ + ⌘ + U to view its Raw Source
- press ⌘ + F to search for the email address prefix Virgin
If the email address prefix Virgin cannot be found then this conclusively shows the issue is not due to what your are sending, i.e. if text similar to the following was found:
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From: "Richard Branson" <richard.branson@blueyonder.co.uk>
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but not text similar to the following was found instead:
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From: "Virgin" <richard.branson@blueyonder.co.uk>
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FYI the 'Use unified mail for this account' is useful when managing multiple email account in webmail, read more here How to manage my email account | Virgin Media Help, and is not applicable to the issue raised AFAICS