The identtcp keyword tells IMTA to perform a connection and lookup using the IDENT protocol (RFC 1413). The information obtained from the IDENT protocol (usually the identity of the user making the SMTP connection) is then inserted into the Received: headers of the message, with the hostname corresponding to the incoming IP number, as reported from a DNS reverse lookup.
The identtcpnumeric keyword tells IMTA to perform a connection and lookup using the IDENT protocol (RFC 1413). The information obtained from the IDENT protocol (usually the identity of the user making the SMTP connection) is then inserted into the Received: headers of the message, with the actual incoming IP number. No DNS reverse lookup on the IP number is performed.
Note - The remote system must be running an IDENT server in order for the IDENT lookup caused by either identtcp or identtcpnumeric to be useful.
When comparing identtcp with identtcpnumeric, the DNS reverse lookup called for with identtcp incurs some additional overhead to obtain the more "user-friendly" hostname. The actual IP number may also be considered more definitive than the hostname returned from the DNS reverse lookup.
The identnone keyword disables this IDENT lookup, but does do IP-to-hostname translation. The identnonenumeric keyword disables this IDENT lookup and inhibits the usual DNS reverse lookup translation of IP number to hostname, and may therefore result in a performance improvement at the cost of less user-friendly information in the Received: headers. identnone is the default. These keywords are only useful on SMTP channels that run over TCP/IP.