==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.4.0
- Deployed with iRedMail Easy or the downloadable installer? Installer
- Linux/BSD distribution name and version: FreeBSD 12.2
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MariaDB
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I followed the "Auto learn spam/ham with Dovecot imap_sieve plugin" https://docs.iredmail.org/dovecot.imapsieve.html documentation.
I get the following error: Cron <root@plankton> /usr/local/bin/bash /etc/dovecot/sieve/scan_reported_mails.sh
From Cron Daemon <root@plankton.massat.net> on 2021-04-28 19:10
Details
mv: illegal option -- t
usage: mv [-f | -i | -n] [-hv] source target
       mv [-f | -i | -n] [-v] source ... directory

In the file scan_reported_mails.sh there is this code section causing the error:

# If there're a lot files, direct `mv` command may fail with error like
# `argument list too long`, so we need `find` in this case.
if [[ X"${KERNEL_NAME}" == X'OPENBSD' ]]; then
    [[ -d ${SPOOL_SPAM_DIR} ]] && find ${SPOOL_SPAM_DIR} -name '*.eml' -exec mv {} ${SPOOL_LEARN_SPAM_DIR}/ \;
    [[ -d ${SPOOL_HAM_DIR} ]]  && find ${SPOOL_HAM_DIR}  -name '*.eml' -exec mv {} ${SPOOL_LEARN_HAM_DIR}/  \;
else
    [[ -d ${SPOOL_SPAM_DIR} ]] && find ${SPOOL_SPAM_DIR} -name '*.eml' -exec mv -t ${SPOOL_LEARN_SPAM_DIR}/ {} +
    [[ -d ${SPOOL_HAM_DIR} ]]  && find ${SPOOL_HAM_DIR}  -name '*.eml' -exec mv -t ${SPOOL_LEARN_HAM_DIR}/  {} +
fi

To resolve this error FreeBSD needs to follow the OpenBSD path.