Dovecot memory limit issue

The issue may occur when dealing with a large IMAP folder (over 50-100K of email messages), the logs would indicate that the IMAP SORT command completes successfully but FETCH fails.

The reason of the issue is insufficient memory limit set (vsz_limit for Virtual Memory Size) in Dovecot configuration. It is defined in /etc/dovecot/conf.d/10-master.conf file. Default value is 256M, it usually is commented out:

service imap {
...
  #vsz_limit = 256M
}

or set explicitly as follows:

service imap {
...
  vsz_limit = 256M
}

To correct the issue, set the value to at least 1Gb:

service imap {
  ...
  vsz_limit = 1G
}

Our primary server currently has it set to 4Gb. Restart Dovecot to apply changes:

service dovecot restart