Hello: I am using a brand new MacPro Tower and a 2009 Imac at home. I experience the same issue on both computers which both run Lion, the latest release, 10.7.4. The issue is with me as well as my co-workers. Our workplace uses Microsoft Exchange server 2010, and using Mac Mail 5.2. We can log in fine using our passwords, then at some time in each day, several times, we stop getting mail. I can tell because its still coming to my Iphone and Ipad. This is strictly a desktop issue. Once I realize this has happened, I have to FORCE quit mail and restart it. Once restarted, I have all my new mail there, as well as any mail that I sent out up until that point that day which is again, open and on my desktop. I can check my outbox and see that they were all sent out at the time I wrote them so not sure why they are opening up as if they were never sent out. I also have to delete previous emails that I deleted earlier in the day. I can then work along pretty good, until I notice again, that I have to force quit and restart. I am not the only one this is happening to in our office, but there are some people that this is NOT happening too and we all use the same equipment. Our techs cannot figure it out. Anyone else having this sort of issue? I truly appreciate any and all suggestions.