[hannob]

You should be aware that Microsoft's SMTP servers parse the mime structure of the mails and restructure it in a non-standard way. I have no idea why, but for example this breaks PGP signatures.

[rockwotj]

This is due to a well known fact that Microsoft hates the email ecosystem.

I work on a email client and a large majority of "this email is broken" is due to weird outlook behavior. Most recently it's TNEF attachments: https://en.wikipedia.org/wiki/Transport_Neutral_Encapsulatio...

[Avamander]

They break way more than that, it's quite annoying. If you search in various MTA/client mailing lists you'll find a bunch of threads.

Though, others do violate standards as well, but not like this.