==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.6.4
- Deployed with iRedMail Easy or the downloadable installer? Installer
- Linux/BSD distribution name and version:  Ubuntu 22.04.3 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  MariaDB
- Web server (Apache or Nginx): nginx 1.18 built with OpenSSL 3.0.2 (TLS/SNI enabled)
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

In my iRedMail instance, users are able to login with webmail, but mail clients are not (App Mail; Outlook;... etc). Most of the time, they faced "cannot connect to POP server" error. Port 110 is opened.

Sample configuration that doesn't work:
POP3's port 110 (TLS checked)
SMTP's port 587 (TLS checked)