Hi, David

1. Does the message contain an embedded attachment, like a Word doc or PDF? If so, that may be the issue. Instead of embedding the file, can it be sent as an attachment?

2. If it IS an attachment, is the file type blocked? For example, is it listed as a blocked extension for her domain? If so, or if there's an * in the Inbound Extension Blacklist, remove the file extension or the * entirely from the list.

3. Where does she see the message about the MAPI 1.0 embedded message? Is it in webmail or does she see this when opening the message in a client? If it's in webmail, have her try viewing the message in either Outlook or another client. If she's using a client that's connecting via IMAP or POP, try having her set up a MAPI account instead. (You can start a trial of MAPI & EWS at the system level if need be.)

4. When you say they're getting "rejected" -- is the mail server bouncing the message or is the message received, but the "MAPI 1.0" text is shown in the contents of the message? 

5. If she forwards to a different mail account, like a Gmail account, is the message received properly? 

Is this a variant of the unusable winmail.dat attachment problem?

I believe that the affected messages are probably related to delegated tasks. Microsoft systems, both clients and servers, allow a user to create a new task and assign it to someone else. That task is then shared with the assignee by sending an email with a specially formatted winmail.dat attachment. The same format is used the task creator makes any changes to the task, or to report back the assignee's progress.

The message that David mentioned is an indicator that the email appears to be one such email.

Based on Derek's reply, I asked her more questions. I'll reply back to this thread when I hear back and know more.