Roundcube error «Failed to send FETCH command»

Symptoms

The error «Failed to send FETCH command» is displayed in any Roundcube mailbox.

Also, the following error appears in the system log at /var/log/maillog (for Debian-based systems the path is /var/log/mail.log):

dovecot: imap(info@maildomain.com): Error: mmap() failed with file /var/www/<user>/data/email/maildomain.com/info/.maildir/dovecot.index.cache: Cannot allocate memory
dovecot: imap(info@maildomain.com): Fatal: master: service(imap): child 28496 returned error 83 (Out of memory (service imap { vsz_limit=256 MB }, you may need to increase it) - set CORE_OUTOFMEM=1 environment to get core dump)

Solution

Increase the vsz_limit and default_vsz_limit parameters in the Dovecot mail server configuration file /etc/dovecot/conf.d/10-master.conf. The default value is 256MB.

After making the changes, restart the Dovecot service with the following command:

systemctl restart dovecot