The mail/postfixadmin port is available as a package but can only be used if Postfix is compiled with SQL support. Since there is no Postfix package with SQL support it means that, even though you can install the mail/postfixadmin package, you cannot use it unless you build Postfix from ports. This does not seem logical to me.
The ideal would be to have one flavor for each SQL knob, but I would already be happy with any SQL since the three of them can be used with Postfix Admin.
Over to new maintainer