==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.5.2 MARIADB edition
- Deployed with iRedMail Easy or the downloadable installer? Download
- Linux/BSD distribution name and version: Ubuntu 20.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): Mysql
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? Yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====
i am using mdbox format.

Couple email accounts get their cache file growing up to 250Mb size. I get following error in /var/log/dovecot/imap.log:
Fatal: master: service(imap): child 3464478 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)

i made changes in file /etc/dovecot/conf.d/10-master.conf
default_vsz_limit = 0
and
service imap {
  # Most of the memory goes to mmap()ing files. You may need to increase this
  # limit if you have huge mailboxes.
vsz_limit = $default_vsz_limit

  # Max. number of IMAP processes (connections)
  #process_limit = 1024
}
restarted dovecot, but still getting error above.

If i delete the cache file in user's INBOX directory, emails can be loaded and read. But after around 1 week, the cache file grows again up to 250Mb and errors pop up.
-rw------- 1 vmail vmail 243M Jan 25 10:07 dovecot.index.cache