/usr/sbin/sendmail is in lsb-invalid-mta 4.1+Debian11ubuntu6.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 | #!/bin/sh
echo "Please install an MTA on this system if you want to use sendmail!" >&2
exit 255
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