If you know for sure that smail is lacking TCP network support, you have to get the source. It is probably included in your distribution, if you got it via CD-ROM, otherwise you may get it from the net via FTP.
When compiling smail, you had best start with the set of configuration files from Vince Skahan's newspak distribution. To compile in the TCP networking driver, you have to set the DRIVER_CONFIGURATION macro in the conf/EDITME file to either bsd-network or arpa-network. The former is suitable for LAN installations, but the Internet requires arpa-network. The difference between these two is that the latter has a special driver for BIND service that is able to recognize MX records, which the former doesn't.