[Exim] Verifying sending domain is not local.

We are running exim 3.33 on our external facing mail hubs.

To prevent us being used as an external relay we have
host_accept_relay = 10.x.0.0/16
relay_domains =
"@:partial-lsearch;/usr/exim/files/relay-domains:[127.0.0.1]"

This seems to prevent relaying to/from arbitrary domains, and preventing
users masquerading as company users to send email to external addresses.

However, it is possible to connect to the mail servers externally masquerade
as a ompany address to send inbound email.

i.e. You can connect to the mail server, issue a mail from:
xxx@???, rcpt to: yyy@??? and have the email delivered. I
know I'm missing something, but need a bit of help in verifying if a machine
issuing a mail from: nopworld.com, rpt to: nopworld.com, is a valid machine
within our domain.

Any help appreiated

Thanks

Justin Holdsworth



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