In this article I will show the steps to troubleshoot Synology Mailplus server
SMTP Port Used
Keep the following in mind:
The Synology Mail Plus server uses SMTP port 25 exclusively to negotiate mail transfers with other messaging servers. This port can be configured under service, protocol, SMTP
Under Service, Protocol, Network Interface, the network interface used by Synology MailPlus can be configured .
So open port 25 in firewall rules for this kind of traffic, like with a relay server. Port 587 doesn’t need to open for this kind of traffic.
Access via SSH the NAS
turn on SSH at your own risk and login via the following instruction, click here.
Live view with aggr_maillog
see postfix/smtpd messages
cd /volume1/@MailPlus-Server/aggr_log
sudo tail -f aggr_maillog
TCPDUMP
tcpdump is a data-network packet analyzer computer program that runs under a command line interface. It allows the user to display TCP/IP and other packets being transmitted or received over a network to which the computer is attached.
Install diagnosis tool:
sudo synogear tool
cd /var/packages/DiagnosisTool/target/tool
Run the following command to display the packets and see the communication between the mailservers
sudo ./tcpdump -i eth0 port 25 -v
Postfix queue
Check the deferred queue:
cd /var/spool/postfix/deferred
ls -R
Check incoming emails:
cd /var/spool/postfix/incoming
ls -R
Check corrupted emails:
cd /var/spool/postfix/corrupt
ls -R
Daily report warnings:
- warning: postfix-script
not owned by root: /var/spool/@MailPlus-Server/postfix/.
According to Synology Support those warnings can be safely ignored.
- connect to localhost:12340: Connection refused
problem talking to server localhost:12340: Connection refused
Based on the error messages, your system is reporting that the Postfix smtpd service is unable to connect to localhost:12340. This specific port is used for internal communication within MailPlus Server, specifically for the quota management service to handle recipient limits.
These warnings occur when that internal component is temporarily unavailable or unresponsive. If this happens during an active mail transfer, it may lead to temporary delivery delays, which are often reflected in logs as temporary 4xx service errors.
To resolve this, please try the following steps:
Check for Updates: Ensure that MailPlus Server is updated to the latest version in Package Center to ensure you have the latest stability improvements for all internal services.
Restart the Service: If the warnings persist and you are already on the latest version, try stopping and then restarting the MailPlus Server package in Package Center during a low-traffic maintenance window. This often clears stuck internal processes.
Monitor Mail Flow: Check if these warnings were isolated events. If your inbound and outbound mail flow is currently working normally and the warnings have stopped, no further immediate action is required beyond monitoring.
