[Exim] Exim and addresses with a forward-slash (aka X.400)

After migrating from Qmail to Exim, a customer complained that we no
longer accept addresses with a forward-slash in them. Exim returns "550
Administrative Prohibition".

While we dislike X.400 as much as the next person, apparently the US
government uses it in several of their departments. One example is:

46/lappappraisal@???

Rather than getting the agencies to redesign their delivery scheme, or
telling the customer to go elsewhere, is there a way Exim can accept
X.400 addresses when bound for remote destinations? I wasn't able to
find any helpful reference to X.400 addresses in the docs, faqs, or the
list archive, but I'm probably overlooking the obvious.

Thanks for any input :)

Rich
richs@???