Problem with writing messages to Sent folder - IMAP - Thunderbird 1.5

I installed Thunderbird 1.5 on a new laptop today and set up the profile 
exactly the same as on my desktop machine.

OS - WinXP (fully patched)

The messages send perfectly using SMTP, however when copying the mail to 
my sent folder on my IMAP server the server replies "Permission denied".

My desktop machine still works perfectly so it isn't a permissions 
problem with my mailboxes.

Anyone else experiencing or not experiencing the same problem?