==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.4.2
- Deployed with iRedMail Easy or the downloadable installer? No
- Linux/BSD distribution name and version: debian 10
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- 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'm trying to send email out through SMTP using:
host: hostname of the mail server
port: 25
username: full email address of the inbox
password: password of the inbox

I then got error message:
Command parameter not implemented. The server response was: 5.5.2 : Helo command rejected: need fully-qualified hostname

The hostname I'm using on SMTP is the same name as the name returned by "hostname -f" on server.

I then found the document "Errors you may see while maintaining iRedMail server
" and trying to find the Postfix log, but there's no mail.log file exist under var/log folder. Do you have any ideas?