"A couple of days ago he decided to switch from Thunderbird (the Mac version) to Entourage. And as it turns out, that particular operation is much faster"
I've found the same thing, Entourage is much better behaved than anything else when it comes to IMAP. Tbird/SeaMonkey mail do indeed fetch a whole help of stuff at once, when you get to around ~200msgs in INBOX, things start to slow down when new messenges pop up.
MacBU - The only M$ division that produces decent software
