Hello,
I got a retuned mail from one of the out going mail.
<553 Relay restriction, by one of following check: SMTP Authorization,
relayres.ini, switches.ini.>
Could anyone tell what is this message mean?

[quoted text, click to view]

It means the remote mailserver failed to accept mail for the domain in
question either because (a) you have been deliberately blocked due to
a suspect IP or other SMTP envelope-level factors, or (b) due to
misconfiguration on the remote end (i.e. a published MX that doesn't
"know" it's an MX and does not in fact accept mail for that domain).

[quoted text, click to view]

It means the remote mailserver failed to accept mail for the domain in
question either because (a) you have been deliberately blocked due to
a suspect IP or other SMTP envelope-level factors, or (b) due to
misconfiguration on the remote end (i.e. a published MX that doesn't
"know" it's an MX and does not in fact accept mail for that domain).