Symptom: Getting any 1 or more of these 3 messages:

Your message did not reach some or all of the intended recipients. or The following recipient(s) could not be reached: or No transport provider was available for delivery to this recipient.

Problem:

The recipient listings in your personal address book are pointing to exchange servers that have been migrated over to other exchange servers; and are thus no longer relevant addresses. When you type in their names, instead of the outlook client grabbing their exchange server address out of the global address book it is getting its exchange server address for them from your personal address book listing for them. You can verify this by right clicking on their name in your personal address book and comparing that exchange server address with what it is supposed to currently be.

Solution:

EASY FIX: Go into the personal address book and remove all listings there.

DIFFICULT FIX: Right click on each person who can't receive your emails and put in their current, correct, exchange server address.