To harden your MailPlus Server enable SMTP authentication on the Submission port 587 and disable auth on SMTP port 25.
Outbound SMTP
First you need to make sure your apps and clients can still send mails.
Go to DSM → MailPlus Server → Services tab
- Enable SMTP with port 25
- Enable SMTP-STARTTLS with port 587
Do not forward TCP port 587 from your router or firewall. Make sure it can only be accessed from your LAN.
Now configure your mail clients to use SMTP port 587 for outbound sending, with (auto) TLS/SSL enabled.
Inbound SMTP
SSH into your NAS.
Open the main.cf template file that will survive restarts:
sudo nano /volume1/\@appstore/MailPlus-Server/etc/template/main.template At the bottom add this next line. It will globally disable authentication, but the lines for port 587 in the master.cf will enable it for outbound SMTP.
smtpd_sasl_auth_enable = noThen restart MailPlus Server to recreate the configuration files and apply the change. This can take a while to complete.
sudo synopkg restart MailPlus-ServerYou need to redo these steps each time MailPlus Server is updated.