A friend uses Yahoo for his email even though he is a Verizon customer. Frequently when he sends or responds to emails from my Verizon email account, he gets Failure Notices saying that the "Verizon Server is unavailable for too long". How can this be fixed short of saying "stop using Yahoo"?
The Yahoo/Verizon issue happens when Yahoo blocks Verizon's servers or vice-versa, and is usually cleared up within a day or two.
You can report it to Verizon if you want, but odds are they're already aware.