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filter that checks if sender domain resolves
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This filter performs a DNS lookup on the domain of the sending e-mail address to determine if it exists before accepting it.


The filter currently supports:

  • performs a Host lookup on the sender address


The filter is written in Golang and doesn't have any dependencies beyond standard library.

It requires OpenSMTPD 6.6.0 or higher.

How to install

Clone the repository, build and install the filter:

$ cd filter-checksenderdomain/
$ go build
$ doas install -m 0555 filter-checksenderdomain /usr/local/bin/filter-checksenderdomain

How to configure

The filter itself requires no configuration.

It must be declared in smtpd.conf and attached to a listener:

filter "checksenderdomain" proc-exec "/usr/local/bin/filter-checksenderdomain"

listen on all filter "checksenderdomain"
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