DC wrote:

It seems to be mostly when I get email from Mac users.

Outlook seems to be fairly unforgiving when it comes to variations in the
message headers. Perhaps the Macs are adding something Outlook doesn't
expect or removing something it can't live without and confusing it as to
where the parts of the message occur. It may help if yiou could post the
complete headers (changing IP addresses and mail addresses if needed to keep
things anonymous) and perhaps someone will spot the difference.
--
Brian Tillman