You can set up Exim to accept all incoming email, and then blackhole (that is, silently discard) it.
Add this as your first router:
blackhole_incoming:
driver = redirect
data = :blackhole:
Alternatively it might be good to send be a warning to your server doesn't accept email, and who should be contacted.
For that you can set up a router and transport like this, but please heed the warning at the end:
auto_reply_router:
driver = accept
no_verify
no_expn
transport = auto_reply_transport
auto_reply_transport:
driver = autoreply
from = "postmaster@example.com"
to = ${sender_address}
subject = "Auto reply something something"
text = "Thanks for mailing us, but we don't accept messages here. Please email foo@example.com for support"
WARNING: This will mean anyone can email your server and cause an auto-reply to be sent to any innocent third party.
If you want to do this, use the "condition" option to the router, to apply a condition which makes sure the person being email actually received a mail from you recently which they're replying to. You can do a database lookup here to achieve this if you store your outbound mail records in a database.
