I have just written a filter that I use with Exim to reject outgoing mail if it appears that I had intended to include an attachment, but forgot to do so.
I hope this is useful to others too! Please find the filter at this address:
http://www.malcolm.id.au/files/software/unix/fta.filter
--
Jeremy Malcolm PhD LLB (Hons) B Com
Internet and Open Source lawyer, consumer advocate, geek
host -t NAPTR 1.0.8.0.3.1.2.9.8.1.6.e164.org|awk -F! '{print $3}'