@gre.ac.uk vs. @greenwich.ac.uk
The University email system will accept incoming messages addressed to you@gre.ac.uk and you@greenwich.ac.uk (where "you" is your username (ab123) or long-form username (a.n.other)).
However, when messages are sent from your account, they use the full Greenwich address (e.g. you@greenwich.ac.uk). Bear this in mind when subscribing to mailing lists that allow posts directly from your email. Make sure you sign-up with your full @greenwich.ac.uk address to avoid your posts being rejected.