[Exim] help re-writing invalid addresses?

I've been trying to re-write some dodgy "From:" lines, which contain the "#"
character.

I read about "SMTP-time" re-writing ( the S flag ) and noted that it said it
took place before any checking and was pleased with my first attempt at a
regexp to match it.

I also thought I'd re-write the addresses in the envelope ( the E flag ).

Sadly, exim rejects the addresses "domain malformed or missing" before I can
re-write them. Can I get around this anyhow?

We've discovered that versions of exim around the 1.9x region accepted "#" so
have a workaround (yuck!)!

EG - I'd like to rewrite "user@dom.#22.other.stuff.top" into "user@???".

TIA,
Steve