OS: Ubuntu Linux 18.04.5, Webmin: 1.973, Virtualmin: 6.15, Usermin: 1.823, Authentic Theme: 19.73
I have a few VPSs (all of them uses the same OS) and they all have the same Virtualmin+Webmin versions installed. The Webmin and Virtualmin settings too are the same on all of them.
For some weird reason, one of the servers can’t connect to external SMTP servers even though I’ve checked and confirmed that the Postfix and Dovecot settings are set up in the same way as on the other servers.
I have a dummy WordPress site on the server that I have configured to send mail via SMTP (manually setting SMTP up by configuring wp_mail as well as using an SMTP plugin) but test mails sent from the site are not delivered.
However, when I do the same thing (using wp_mail or SMTP plugin) on live WordPress sites hosted on the other servers, the test mails are delivered without a problem.
Running “re-checking configuration” returns no errors and I tried checking the mail logs in
/var/log/mail.log but there isn’t anything useful there too other than a few lines about brute-force login attempts (SASL LOGIN authentication failed).
What could be causing this particular server to be unable to connect to external SMTP servers?