For remittance advices, where is the email address picked up from?

We've got a situation on eBusiness R12.1.3 where some remittance advices are being emailed out to suppliers, and some aren't.

There are a few things I can think of where it may be going wrong.

1) clients exchange server is the source of the error (i'll ask the client to check this)

2) some of the suppliers email addresses aren't correct - for this can anyone confirm definitively where it picks the email address from - I've searched through some documentation and have found a couple of potential tables, but if anyone can confirm for sure where it's pulled from I can check those tables.