Dear Maintainer,

I installed ecryptfs-utils. This modified the files in /etc/pam.d/ by
adding the line:
session        optional        pam_ecryptfs.so unwrap

Somehow this line broke unix authentication that is used by Exim. This
caused that I couldn't send mail anymore since the mailserver would
not accept mail. Unfortunately it made my mail client time out and
made Exim hang without error in the log and leaving me totally
clueless on why suddenly Thunderbird/Exim stopped working.

Starting Exim in debug mode showed that its last action was calling
PAM. That made me check out /etc/pam.d/ and thanks to etckeeper I
could figure out what had changed, which pointed me to ecryptfs.
Uninstalling the package fixed my problem.