Re: ftp://ftp.porcupine.org/mirrors/postfix-release/LICENSE

  I'm going to add it as pkgsrc/licenses/postfix-license

That's perfectly fine.

  and as it looks quite reasonable should it be added to
  DEFAULT_ACCEPTABLE_LICENSES as well?

The documented policy is to add licenses if they have been approvaed as
Free (FSF) or Open Source (OSI).

However, we haven't yet enabled license.mk so while adding
postfix-license is great, you should leave LICENSE=postfix-license
commented out, assuming that this license is at least 99% open/free.