- Allow incoming trafic (only) to MDaemon, WorldClient, WebAdmin, XMPP and WCXMPP apps on this and that port
I'm happy to help define rules for your firewall. Based on what you have already told us, it looks like you have a pretty good set of rules, however, there may be some minor tweaks that would help to improve security.
Are there any services in MDaemon that are running and that you have ports open for that you are not using? For example, XMPP, POP3, or Minger? If not you are not using the service then don't allow the traffic on those ports and turn off the servers in MDaemon. This helps to reduce the attack surface.
Is anyone using the IMAP and SMTP SSL Ports? Typcally clients today favor the standard ports and using TLS so you may also be able to close the SSL specific ports.
Is the ODMR (366) port being used? If it is being used to allow users to send mail on an alternate SMTP port, transition users to the MSA port (587). You can't turn off ODMR in MDaemon, but you can stop allowing the traffice on that port at your firewall. If you are not using at all, then don't allow the traffic from the firewall.
But my question was not if we can have something like that, rather what can I do more to use existing resources to enhance security.
Enable password policies in MDaemon to force users to use strong passwords and force them to change their passwords. If you are still using the option in MDaemon to store passwords in the userlist.dat file, turn on the option to store passwords securely. Accounts / Account Settings / Passwords, Store mailbox passwords using Non-Reversible Encryption.
Enable the option to check 3rd party databases for compromised passwords. Accounts / Account Settings / Passwords, Do not allow passwords found in third party compromised password lists.
Use App Passwords.
If users are using webmail, force them to use two factor authentication.
If you do not have a need for Remote Administration access from the internet, close the ports at the firewall and only allow traffic from the LAN to Remote Administration. If you want to allow Remote Administration access from the internet, restrict user access as much as possible and force all Administrators to use 2 factor authentication.
Force the use of SSL and TLS all the time.
- AV scanning/realtime protection to exclude only folders processed by MDaemon and therefore protected by Ikarus/ClamAV. So not complete \MDaemon folder but rather (for example) subfolders
We reccomend excluding the entire MDaemon directory structure. MDaemon expects to have full control of all files in its directory structure, even the EXEs and DLLS. If you don't, it can result in unexpected behavior. For example, if you have a 3rd party AV engine scanning and it decides something that MDaemon.exe is doing looks malicious, it might stop the process and quarantine the EXE, which will ultimately result in your mail server not functioning. If the 3rd party AV engine tries to quarantine the IKARUS EXE or DLL after IKARUS has updated itself and prevents MDaemon from starting its AV scanning, you could be left with a running mail server that can't process any mail.