We had this wierd problem with a user who was using Entourage on his desktop mac and also his macbook when he was out and about.  The desktop mac worked fine all of the time.  It doesn’t syncronise sent items, however thats a separate issue.  The problem with the macbook is that on one particular network, it would send emails for a period, then refuse to send or receive emails.

Usual trrouble shooting: Change router, Packet sniffing, Check webmail etc, etc – you name it, we tried it.  We were on the point of purchasing the guy a PC for use exclusively when he was in this particular office.

I decided to call the email provider one more time and run through the problem with them.  The one thing we noticed was the way the url for exchange was entered.  The instructions for their service were for Exchange 2003.  They had recently upgraded to Exchange 2007.  The URL string should use the full email address rather than the numeric mailbox number.  I was sceptical that this would have any impact, however we made the change and guess what?  The darn thing works all of the time ….

So, ensure your connect string for exchange is something like this if you are using Exchange 2007:

https://exchange.simplyms.com/exchange/user@userdomain.com