A while ago I worked at a site where I was installing Exchange 2007.
Everything was great, except occasionally, Blackberry devices would reply to a mail but the recipients the reply went to would not see any reply!
Well this is some text from a Blackberry support agent:
It appears the cause of this issue is the modification of the PR_RTF_COMPRESS property of the email message which prevents the BlackBerry Enterprise Server from determining how to format the reply message.
What we have seen in a few other cases such as this is that an SSL Appliance has modified this property before the message arrives on the Exchange server.
To workaround this issue for now you can create the a DWORD Registry key named AllowRepliesInHTML? under HKLM\SOFTWARE\Research In Motion\BlackBerry Enterprise Server\Agents on the BlackBerry Enterprise Server and set the value to 0 (zero).
Hope that helps someone!