Because people had different ideas at different times, we have used 3 different methods for sending email in Cloverleaf programs. Two are executable Windows programs: bmail.exe and POSTIE.EXE. We “exec” these in Tcl code. I wasn’t around at the time so I don’t know much about where those programs came from.
More recently, we have also used an all-Tcl method for sending email. Tcl comes with libraries for email (and lots of other things). The very basics look something like this: