SMTP is the simple outgoing mail protocol. Verizon used to have a "public" SMTP server which you could send messages through, but no longer has it. I've had issues using Charter over an EVDO modem as well. What worked for me is using GMail over IMAP (a more robust, feature-rich mail protocol than SMTP). This way, you can send/receive through GMail which can, internally, send/receive through Charter.

Good luck.