We have 2 Exchange servers in our environment, server A and server B. When users with mailboxes on server A send attachments to other users with mailboxes on server A, they go through without a problem. Same with server B to server B. When a user sends from server A to server B, with large attachments, they go through without a problem. When a user on server B sends to a user on server A, with large attachments (usually 6mb or larger), the attachments get converted to winmail.dat files. We have rich text format turned off at the Exchange level so we know it's not that. What could be causing this?

Hi, 1. Would you please let me know whether the two Exchange Servers belong to same organization or different organization? 2. Whether any Anti-spam/Anti-virus software installed on the two Exchange Servers? If yes, I suggest you temporarily disable the Anti-spam software to check the result. Based on my research, the Scanmails anti-spam and Trend Micro may cause the issue. Thanks Mike

Both servers are in the same organization. We are currently running Trend Micro ScanMail for Exchange on both. Why would the anti-virus convert the attachments going from server B to server A to winmail.dat but not from server A to server B?

Hi, I notice several cases in our internal database have similar features and the issues related to Trend Micro scanmail. Nevertheless, currently, it is hard to say whether your issue is also caused by Trend Micro scanmail. Therefore, I recommended you temporarily stop Trend Micro service and restart the SMTP service to check whether the issue persists. If stop Trend Micro service is able to solve the issue, I recommend that you contact Trend Micro support regarding the issue. Here are some information I searched from Trend’s website: [Hot Fix] B1235 - External recipients or those whose mailbox are not on the same server receive their unreadable email with "winmail.dat" as an attachment http://esupport.trendmicro.com/support/viewxml.do?ContentID=EN-1036704&id=EN-1036704 A second email with only a winmail.dat file attachment is received when sending an email through ScanMail for Exchange (SMEX) 8.0 http://esupport.trendmicro.com/support/viewxml.do?ContentID=EN-1037111&id=EN-1037111 Thanks, Mike