Any SMTP server that is not listed as a public Mail Exchanger in the DNS zone of your domain(s) should not accept incoming connections from the internet. All incoming mail traffic should go through your incoming mail exchanger(s). .:: docs.google.com ::. .:: orcid.org ::. .:: www.tipntag.com ::. [Debug Laravel] .:: pod.beautifulmathuncensored.de ::. .:: podcasts.apple.com ::.
This consideration is not unique to SMTP servers. If you have machines that only serve an internal purpose within your site, use a firewall to restrict access to these. .:: telegra.ph ::.
This is a rule, so therefore there must be exceptions. However, if you don't know what they are, then the above applies to you. [Open Source Express.js]