You can always, by definition, receive email using just your username with @sussex.ac.uk added to it, such as in
Members of staff and postgraduates normally have a 'friendly address' of the form:
If you have such an address, it's best to use that in correspondence because it's easier for people to remember. Your 'friendly address' acts as a pseudonym, and is in fact resolved down to your username when email is actually delivered to you, though this action is transparent to you and your correspondents.
Please note that undergraduate students do not normally have 'friendly-style' addresses. FAQ 523 explains this in more detail.