How can we reproduce this locally? Do you have a specific version of Outlook / OS you are using?

With the .eml files, you could change the From: and To: headers and pipe directly into sendmail or similar MTA.
If that doesn't work, I would be happy to provide a VM instance to assist in reproduction.

We are presently using Windows 7 and Office 2007

We have no plans to support Windows 7. Do you have issues with more modern versions of Outlook?

Or I should say, the amount of time that would be needed to get to Windows 7 issues, would negate the need to support it. We're two developers and any bug is competing with thousands of others.

Makes sense, these are old versions of software. I will test with more recent versions of both Windows and Outlook to see if the issue persists.

I am also a developer and see if I can find out where the parser is choking. It seems to be an issue with cid: references, but both Outlook and Thunderbird use them. I'll have to see if it's an issue with the MimeMailParser (which we are using via local MTA) or if it's upstream, which means this could affect external mail API consumers as well.

So I've figured out the underlying cause. Outlook is sending attachments without a Content-Disposition header. The current version of mimemailparser included with Phabricator completely ignores attachments without that header.

This problem has been solved in the upstream version of mimemailparser here: https://github.com/php-mime-mail-parser/php-mime-mail-parser/blob/master/src/Parser.php#L392

I have backported the relevant parts to

Minimum viable patch sounds good to me, but @epriestley handles all contribs. Not sure what he prefers here. Feel free to patch locally first and verify.

Yeah, go ahead and submit a patch. Thanks!

Submitted D16584: Backport fix from php-mime-mail-parser to fix attachment parsing