SUMMARY

You may wish to send to an alternate server if the message fails to send to the original recipient server. This redirection to an alternate server will only occur if the recipient server returns a 503 error for all the recipients.  

DETAIL

To do this you need to add the following registry keys and restart the SMTP service (changing example.com to the domain you wish to send to).

On a 64bit Windows server:

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mail Enable\Mail Enable\Connectors\SMTP]

On a 32bit Windows server:

[HKEY_LOCAL_MACHINE\SOFTWARE\Mail Enable\Mail Enable\Connectors\SMTP]

"SmartHost on RCPT failure"=dword:00000001
"SmartHost on RCPT failure server"="example.com"