Suddenly observed emails couldn't reach my email server after setting up completely and working properly for quite a while. Then check the following:
- if the server (Postfix/Dovecot) are up and running. Using Linux CLI to ensure mail servers are good but only internal mails were received;
- any change of configuration files including access, accidentally removed, etc.
- check Port Forwarding set up of the router. Port 993 was properly setup.
Open port forwarding in the new router for port 25. Actually, I misunderstood 2 points:
- even though it's incoming mails, SMTP has to be working properly
- even port 25 is blocked for outgoing mails, but it is still used for incoming mails and should be opened and forwarded to the mail server.
Based on these misunderstanding, I did not set port 25 on my lately purchased router, but thought that only port 993 (IMAP over SSL) is required for incoming mails. For my case, outgoing mails are sending out through Google SMTP as the ISP blocked 25.