"Hello! Would you like to set up Mail now?"
"Yeah, why not? On you go, computer."
"Would you like to use [username]@[my-university.ac.uk]?"
"Yeah, that - wait. How did you know that?"
"I beam at you with my aesthetically pleasing Aqua interface."
"But I didn't tell you that, and I've never tried to set up Mail with that account before, and, come to think of it, I've never even checked that account from this computer. How did you know?"
"Behold my inviting setup window. I can wait as long as you want."
"But how? Seriously. How did you do that?"
"I am helping you! I am being helpful!"
"You're... disturbing me, a bit. What else do you know?"
"Oh, you'll find out, human. You'll find out."
"But I didn't tell you that, and I've never tried to set up Mail with that account before, and, come to think of it, I've never even checked that account from this computer. How did you know?"
If 'my-university.ac.uk' is the physical network you are on, it simply used whatever user name you have plugged in and connected it to the name.
Or, there may be an email directory server on your net.
max
['That trick works only when things are just so. But it's a good trick when it does work.']
Weirdly, it wasn't linked up to the university network at the time (and hasn't ever been). It remains a mystery. Obviously, I must have given it (or the old computer, most of the contents of which are now sitting on this hard drive) that information at some point, but I can't for the life of me remember when.