[Exim] Sender verify - What is check_postmaster?

Hi guys,

    Using Marc Merlin's Debian package for Exim v4.10 w/ SA-Exim.

    I have a sender verify acl in check_rcpt that looks like this...

  deny    hosts = !+localadds:!+host_disable_callback:*
          sender_domains = !+envdomain_disable_callback:*
          local_parts = !+noenvfromcallback
         !verify = sender/callout=90s/check_postmaster

    Can someone tell me exactly what check_postmaster does?  I can't
find it in the Exim docs, and search of the list archives turns up nothing,
except excerpts from Marc's configuration file.

    I assume it checks for the existence of a postmaster account during
sender verification.  Can I do sender verification without this?  We
communicate with a number of independent schools that are typically
understaffed, and have poorly managed/designed networks.  Thus, they usually
run MS Exchange and don't have a postmaster account.

Thanks,
Steven
---
Steven Dickenson <sdickenson@???>
Network Administrator
The Key School, Annapolis Maryland