DSN_RELAYED_FORMAT

(string, 0-256 characters) Formatting string for relay notifications. The default value is the string:

Your message to $a has been relayed to a messaging system which may not 
provide a final delivery confirmation

To inhibit conversion of relay notifications, specify an empty string for this option. Note that GATEWAY_NOTIFICATIONS must be set to 1 for this option to be in effect. This option is ignored when GATEWAY_NOTIFICATIONS=0.