The nssmtpd module is recommended (see e.g.
https://openacs.org/xowiki/outgoing_email), but is not required. We experienced various problems using the mail delivery from tcllib (such as invalid mail-encoding, crashes when more than 1024 file descriptors open, problems with tcltls). I think, these problems can be addressed with the right version combination of tcl, tcllib and tclltls, but i can't give you a recommendation what exactly avoids the problems wem had. We switched on our high mail traffic NaviServer site to the nssmptd module many years ago and our problems disappeared.
