What's weird here, I think, if I'm understanding your description correctly:

- His email address is a Verizon.net address.
- The delivery failure occurs when she's on the Verizon.net FIOS wifi connection.

If I'm understanding it correctly, that's the situation in which the message should arrive with the best chance of being received: With most of the middlemen cut out.

So, though your descriptions seem to indicate that it's getting spam-binned on his end, I'm wondering why the same messages from OUTSIDE the verizon network don't get spam-binned.