However, only Exim can save you in the above scenario,
Well, not only
Exim can save you. Sendmail can do all of this too. User validity checks, spam filtering, dropping after RCPT TO:, are all supported either natively, or in ways that require some minimal Perl scripting under MIMEDefang or Sendmail::PMilter.
If you're using MIMEDefang you can run your Perl code preforked in a separate process and multiplex across them -- no need to have a 1:1 mapping of incoming SMTP connects to Perl processes. I think Sendmail::PMilter can do this, too, but I've not had the opportunity to look at it in detail.
Exim may be a good MTA, but it's not the only choice for doing this sort of stuff.
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