I have recently researched the exact problem. Are you running Norton antivirus on the client? If so, the problem is that NAV will not release a mail with an attachment to the local (Client) offline folder until it has been opened and scanned. There is no workaround for this that I have found yet. Make sure your user knows to open all new mail, esp those with attachements before synchronising.
Clue:
In the Synch log, it will tell you which folder(s) is/are having the problem. If you Right-Click / Properties on that folder, and look at the Synch tab, it will tell you how many mails are in the server folder and how many in the offline. This will give you some pointer as to which emails are holding up the process. Also, mails forwarded as attachements (ie mail within mail) are very poorly handled. you need to open all the nested mail before synch can occur.
