Macbook Pro 2008 won't send mail (v4.6)

My wife is running Mail 4.6 on her early 2008 MBP, accessing an IMAP/SMTP email account of an organisation that we both work on behalf of. I am accessing the exact same account with the same version of mail using an early 2009 iMac. I am able to send and receive mails, whilst she can only receive. We're both on Mac OSX 10.6.8.

I have been through the account setting a couple of dozen times and they are identical, so I'm at a loss as to why she is unable to send mail. Unsent mails come back with an error referring to 'OK Dovecot ready.' which, after checking on the web, I understand means that the mail client is connecting to the server but it somehow all falls to pieces thereafter. Assuming that this is some sort of server connection issue, wouldn't I expect to see it happening on both computers rather than just the MBP?

Any suggestions would be greatly welcome.

Thanks