I moved a great deal of my mail over to Zimbra 5.0 this weekend and the migration went great until I got to one domain where the users had quite a few large attachments. I had neglected to increase the allowable attachment size in Zimbra causing many of the message imports to fail. To up the limit run the following commands as root:
su - zimbra
zmprov mcf zimbraMtaMaxMessageSize 50000000
zmprov mcf zimbraFileUploadMaxSize 50000000