SMTP error code 551

Hi all,
   has an urgent problem that Eventum on FC4 won't send
email to the assignee after i change network ID and hostname
on FC4, before that Eventum still able to send email to the
assignee. i confirm all necessary service is turned on, etc
the Mail (SMTP) service. i have confirm that i put in
correct SMTP outgoing email address in Eventum General Setup
and corresponding project outgoing email, but it just prompt
out that error 551 saying that:

   Eventumlocalhost.com is not permitted to access your email
server.

but i DID NOT enter any Eventumlocalhost.com, what i
entered in the SMTP outgoing is another valid email for my
organization, why izzit always detect Eventumlocalhost.com instead of real email address? any
help? thanks a lot. by the way, i send email through
external server, i got internet access through FC4 server.