I upgraded from 3.1.4 to 4.5.5 over the weekend

Everything seems find except for this

One of my users, her Blackberry through Vodafone in the UK (using pop3) stopped picking up the emails, I can see the connection coming through in the mailbox.log and getting a message "disconnected without quit".

At first I thought I solved it be re-indexing her mailbox, it was a 1.8Gb mailbox, it worked for about 4 hours and stopped again.

I then deleted her mailbox completely and created a new one, again it worked for about half a day and stopped again.

My own blackberry through a different carrier O2 and using imap has no problems. The other difference is I use only the web client as my main client, but she uses Entourage on a mac as her main client.

Can any one help?