SMTP servers (MTA)

There are a few MTA software (SMTP servers) under Open Source licences. But what you found is really complete.
The most widely used are Sendmail and Postfix.
Other possibilities exists with Exim and QMail, but the two first are the best choices.

If the configuration of Sendmail may seems to be a little bit more complex than the one of Postfix, the adjonction of the M4 macros pre-processing has greatly simplified this job since the early times. With its simple configuration file where  parameter = value, Postfix can be more quickly set up than Sendmail. But when your mail server is just a relay and needs to perform some very specific filtering on the messages passing through it, Sendmail and its Milter API may be a great choice, combine with the swiss-knife of mail filtering programs, MIMEdefang.

When reading that, don't think that Postfix is not suitable to do filtering, but Sendmail combined with MIMEdefang allows any kind of filtering to take place, because in MIMEdefang you define any filtering action you want on the mails using Perl scripts.