MTAs are the programs which transport mail from one machine to the other. The traditional MTA is Sendmail, however there are several other choices. BLFS does not include the Sendmail package anymore, but both Exim and Postfix provide the same functionality as well as a compatibility symlink for applications that expect Sendmail to be present.
In addition to the SMTP servers, there is also a POP3/IMAP server (Dovecot).