Look in Tools, Accounts, Mail, Properties, General. Do you have your
correct e-mail address in the e-mail address field? What about the
Reply To e-mail address?
The error could also indicate that you are using the wrong SMTP mail
server or the mail server now requires SMTP Authentication. In OE this
is specified in Tools, Accounts, Mail, Properties, Servers, Outgoing
Mail Server, My Server Requires Authentication.