There are times when users may encounter errors in SWA about not displaying because of invalid MIME Headers.

There could be many reasons for this and usually the Inbox fills with "System Administrator" errors messages, but they are no longer visible in Outlook or any other IMAP Account.

The below solution fixed my problem, if it doesn't fix yours check the Scalix Forums. 

Find out the System Login ID of the user.

# omshowu -n "user name"

You'll get output with....

System Login :  55008

Close all the user's mail clients. Now check for all imap processes

# ps -ef |grep imap 

Look for entries with the System Login ID of the user.

[root@mail ~]# ps -ef |grep imap
root      3092     1  0 Jun08 ?        00:00:01 in.imap41d -d
55008    21644     1  0 Jul10 ?        00:31:18 in.imap41d -d
dovecot  30657 24834  0 Aug04 ?        00:00:00 imap-login
dovecot  30705 24834  0 Aug04 ?        00:00:00 imap-login
dovecot  30884 24834  0 Aug05 ?        00:00:00 imap-login
55005    13129     1  0 08:20 ?        00:00:02 in.imap41d -d

then kill any processes still existing, as you can see above there was a defunct process going back a month to Jul10.

Log back on and the problem should be resolved.