IMAP Attachments/Thunderbird

[r8escjohn]

Have an extremely intermittent issue with Zimbra 5.0.6 FOSS and Thunderbird clients set for IMAP access.
Every once and a while if a user receives an e-Mail that has an attachment, that attachment does not show in the e-Mail received in the Thunderbird however it is listed in Zimbra Web Client and able to be accessed from there. At first I thought it had to do with attachment size, however just now I had a user come to me with this issue and the attachment in question was a 195k PDF file. Just only in the message in the Web Client and not in Thunderbird.
Any ideas where to start looking to track issue down? It is *so* intermittent it is hardly worth mentioning, but wanted to see if anyone had any suggestions or had ever ran into this before. Thanks!