Preventing backscatter with sendmail

[Paul Welsh]

I'm looking for a way of doing with sendmail what exim does out of the box,
ie, recipient verification when the MailScanner/Sendmail server is a front
end to Exchange.

I've been googling and from what I can see I have the following options:

milter-ahead - www.milter.info/sendmail/milter-ahead/
Mailfromd
scam-backscatter - www.elandsys.com/scam/scam-backscatter/
access.db
ldap lookups

Only about 100 - 120 email addresses so access.db is a possibility.
Milter-ahead costs 90 euro (probably worth £150 by the time I can get round
to buying it!).  LDAP lookups seem a bit complex.  

Mailfromd seems the most popular.  Is that the de facto standard way of
doing this?