RE: [Exim] How to deal with 10 fold increase in mail traffic

>1. Spammers sending e-mails from various addresses to various addresses
with the sender's
>address as XYZ@???. XYZ is a random string.

Are the email addresses legitimate? If they are truly random, then could you
not just verify that the users exist before accepting delivery? That way you
wouldn't then need to accept the mail and then bounce it.

Something like:

  accept  domains = +local_domains
          endpass
          message = Unknown local user
          verify  = recipient

In your SMTP ACL should do the trick.