[exim] "Bad words" filtering

We're currently using a custom written smtp server that filters "bad words".
I'm thinking that I'd much prefer using a generic MTA such as exim or
postfix, my personal preference being exim because I think it's much more
flexible. I'm just wondering what people think would be the best way of
achieving this.

I'd prefer to have the rejection at smtp time rather than generate a bounce
and I was thinking a few possible solutions might work.

a. acl_smtp_mime with mime_regex containing the "bad words" for things with
the appropriate $mime_content_type. The disadvantage of this being that it
only captures the first 32K and its CPU-intensive nature.

b. Add the badwords into spamassassin rules and search $spam_report
for matches on these rules. It is a bit of an abuse of spamassasin but it
should work.

c. Pipe the message through a filter transport. This is obviously my least
preferred option as it will generate bounce messages after mail is accepted
or effectively blackhole the message if the bounce is supressed.