/etc/pam.d/esmtp is in courier-mta 0.76.3-5+deb9u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | # PAM configuration file for Courier IMAP daemon
@include common-auth
@include common-account
@include common-password
@include common-session
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