I said:
A 550 error is an SMTP transport error. It does come from his ISP.
A correction. It is an SMTP error, but it doesn't necessarily come from the
recipient's ISP.
Gary said:
What does this error mean and how to do I correct?
A 550 error is often "relaying denied". It is a fair indication that you
aren't properly authenticating to your outbound mail (SMTP) server or are
using a domain that does not match your ISPs domain. For example, people
using SBC Yahoo! often have a domain of "sbcglobal.net", but they use one of
Yahoo's mail servers (smtp.sbcglobal.yahoo.com) to relay their mail. Since
"sbcglobal.net" doesn't match "yahoo.com", ordinarily, Yahoo! would reject
their mail with s 550 error. However, if SBC Yahoo! users check the option
to authenticate to the outbound server, Yahoo! realizes that they are valid
members and allows the relaying of the message from the sbcglobal.net domain
through their mail servers.