@morrow without logs this will be hard to track down. can you ask your administrator to open up a support case for this?
To offer a better compatibility for users with generic IMAP clients like Mac Mail and Thunderbird, the Zarafa Gateway is rewritten in ZCP 7.0. The new IMAP gateway offers a new configuration option to store more message information, like the envelope information, body structure and whole RFC822 message. By storing this extra information, the gateway doesn’t need to do a conversion from MAPI to RFC822, which will speed up the gateway tremendously.
Not only a major speed improvement is realized with this option, but also the following issues will be solved: