I have found a workaround for this, but not a solution
On my computer it is when the email address, mine or the person I am sending to is in quotes.
If I right click on the contact and copy the pure address rather than the one in quotes it works fine. so Jack@aol.com works but "jack@aol.com" bounces.

But this is really annoying; I'm not techie enough to understand how to fix it.