550 5.7.1 Unable to relay for name@domain.com System.net.mail

one day, the ASMX web service can’t send email any more. When I run a cordbg and attached to the W3wp. turn on to catch all unhandled exception, “ca e”

first, get CDO Exception. then continue, get the error like

550 5.7.1 Unable to relay for name@domain.com

then I change different TO Address, no luck , still the same error.

the Service just a wrapper of  the System.net.mail to send email , and uses a local SMTP server.

it looks like that’s a fat finger issue. When I turn on the Relay setting of the smtp server.

I get the following settings as shown.

which means that all relay will be denied. unless you specify it in the exception list.

then just switch it back.