Problem: IMAP-login fails with "Too many open files"

Problem
Users in a larger Dovecot-based IMAP-Environment are unable to logon.

Investigation
In /var/log/dovecot.err messages like the following appear:

Apr 11 11:11:11 imapserver001 dovecot: imap-login: Error: net_accept() failed: Too many open files
Apr 11 11:11:11 imapserver001 dovecot: imap-login: Error: socketpair() failed: Too many open files

Explanation
The default limits are too low for this usecase.

root@imapserver001:~# systemctl show dovecot.service | grep -i limitno
LimitNOFILE=4096
LimitNOFILESoft=1024

Solution
Adjust the limit by adding an override configuration to an appropriate value or to “infinity” (unlimited).

use:

systemctl edit dovecot.service

and add the following lines

[Service]
LimitNOFILE=infinity

Note: this creates or edits the file /etc/systemd/system/dovecot.service.d/override.conf

root@imapserver001:~# systemctl show dovecot.service | grep -i limitno
LimitNOFILE=18446744073709551615
LimitNOFILESoft=18446744073709551615

You will have to run service dovecot restart after changing these values

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