SUMMARY

The SMTP service can redirect failed mail to an alternate server when the RCPT command fails. The settings outlined in this article are non-standard (unsupported) and should only be changed where there is a specific requirement to change the behaviour of the SMTP connector. The redirection will only occur if the recipients server returns a 503 error on the RCPT command. The reason for this restriction was originally for certain configurations of MailEnable servers, but it may be useful in other cases.

DETAIL

This customized behaviour allows MailEnable to attempt delivery via an alternate server should the SMTP recipient command fail when sending from the primary MailEnable Server.

Consider any message looping or undesirable behaviour that could arise from having mail re-routed to an alternate server.

The registry settings to change the SMTP Connectors behaviour follow:

Key Root: HKEY_LOCAL_MACHINE\SOFTWARE\Mail Enable\Mail Enable\Connectors\SMTP
Key Name: SmartHost on RCPT Failure
Key Type: DWORD
Key Values: 0,Missing=Disabled; 1=Enabled

Key Root: HKEY_LOCAL_MACHINE\SOFTWARE\Mail Enable\Mail Enable\Connectors\SMTP
Key Name: SmartHost on RCPT Failure Server
Key Type: REGSZ (STRING)
Key Values: [String: IP Address or Host Name of Server to route mail to)]

Example Registry Import File:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Mail Enable\Mail Enable\Connectors\SMTP]
"SmartHost on RCPT Failure"=dword:00000000
"SmartHost on RCPT Failure Server"="mail123.mailenable.com"

This setting can be useful if the server is prone to blacklisting. The effectiveness of the setting depends on the action taken by the target mail server when it detects the server as being blacklisted or banned.

Some mail servers will immediately refuse connection and others will deny access at the RCPT command (in which case this setting will cause the mail to be routed).  But, if a mail server refuses connection at any other stage in the SMTP transaction, the setting will have no effect.