Not sure if this is only on Macs but it is an infuriating “feature” and sure to cause complaint at some point as more users get Outlook.  Long story short, if you highlight a portion of an email and then forward it or respond to it, ONLY the highlighted portion shows up in your email.

The only fix is “don’t highlight” – sigh.

outlook.uservoice.com/forums/293343-outlook-for-mac/suggestions/10081041-option-to-disable-quote-only-selected-text-on-rep