This is the most important part. Exchange (due to its history as an X.400 server, not as an SMTP server) does sometimes mangle the message to the point that DKIM simply breaks. This both breaks origin-incoming and forwarded messages.
BTW, Apple also sometimes mangle messages that it fails DKIM, although I do not know why is this the case (as I doubt they use Microsoft Exchange for their mail service).
This is the most important part. Exchange (due to its history as an X.400 server, not as an SMTP server) does sometimes mangle the message to the point that DKIM simply breaks. This both breaks origin-incoming and forwarded messages.
BTW, Apple also sometimes mangle messages that it fails DKIM, although I do not know why is this the case (as I doubt they use Microsoft Exchange for their mail service).