How to solve the "The message could not be sent because it has unresolved recipients." in Outlook for MacOs?
Looks like some destination e-mail address is wrong formmated (like: missing domain, or just some name without full e-mail address) 

I would check all the destination recipient address and validate them for syntax. It the case for checking syntax name@domain.com (formatting syntax). 

This is not because non-existing final destination mail, is just technically wrong formatted e-mail syntax addresses

Those messages seems to be on the outbox to be sent, stucked there because of that

Check Outlook outbox for queued messages to fix that.