Malware

Outlook can be picky when it comes to variations in the
message headers,while most smart phones are smarter than that.

To clarify, it's not encrypted or decrypted, is encoded or un-encoded. There is a line in the message header that says something like:

Mime-Version: 1.0 (Apple Message framework v623)

Your problem may be tricky to fix, because the real problem is that the sender of the email is adding something odd to the message header. More specifically, they are sending an email with a MIME encoding scheme invented after 2003.

There may be a service pack for Outlook from Microsoft that fixes this.