I called Verizon's Tech support and they incorrectly told me to update my Airport software - "My colleagues are telling me its version 2.0.4." I later found out that Verizon is using an MTU setting of 1492 instead of the standard MTU 1500. You can set the MTU manually in the terminal by typing:
% sudo ifconfig en1 mtu 1492Doing this is only good until you reboot your Mac then it defaults back to 1500. Here's my solution.
Log in as root and launch the terminal. I used pico but vi and emacs will work, too. Open the network file located at /System -> Library -> StartupItems -> Network -> Network and find the references to setting up your network interface. Add a line and enter ifconfig en1 mtu 1492 for airport or ifconfig en0 mtu 1492 for ethernet.
This needs to be done on ALL computers connected to the internet and sending email using Verizon.net. Restart your computer and you should now automatically be setup with an MTU of 1492 and emails should send correctly. Life is good.

