More unusual spam

This time, maybe just a case of mistaken identity, sent from a CPA firm in Oklahoma

I am using our domain name (IP Address), my user ID, and password, but still can't access webmail. I am using the following form:
http://IPAdress.com/webmail
and on the login screen, I use my user ID and password, as example:
SBCUserID
Password

I can't get in to manage my account... What's wrong?

I replied, saying he probably is asking the wrong person, but couldn't resist this dig:

p.s. SBC is a notoriously poor ISP, my company dropped them after 30 frustrating days.