libmilter-dev - 8.15.2-14~deb10u1 main

The Sendmail Mail Filter API (Milter) is designed to allow third-party
programs access to mail messages as they are being processed in order
to filter meta-information (headers) and content.
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The Debian Sendmail package is built to support libmilter.
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This is the libmilter static library and include files for development.
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The documentation on building a Milter is in the sendmail-doc package.