It's confirmed that this is a "bug" in iRedMail, it doesn't set "WANT_BDB_VER" in /etc/make.conf.
You can set "WANT_BDB_VER=48" in /etc/make.conf before installing iRedMail, then only BDB-4.8 (/usr/ports/databases/db48) will be installed.
Since this is an installation issue, i re-packed iRedMail-0.8.4 and it's now available here:
http://www.iredmail.org/download.html