I recently discovered a weird bug in OS X and macOS Mail app: if you put an attachment at the beginning or in the middle of your message, Outlook users will see the message cut off. This has to do with how Outlook handles rich text messages and attachments. When Outlook finds an attachment in the middle of a message, it recognizes the attachment as the end of the message itself, truncating everything that comes after that.
Solution for the recipient
If your message is cut off, you will see that you will have an additional attachment in txt format. Opening it will reveal that it contains the missing message, albeit without formatting.
Solution for the sender
Do not add your attachments right away. Instead, first write your message and then use the Attachment button on the upper right corner of the message window, as shown in the screenshot below.
This bug has been known to exist since at least 2009 and it appears that Apple has no intention of fixing it.