You may be experiencing timeouts in your mail engine, this was mentioned in another recent topic. Postfix apparently has a 5 second timeout by default.
It took me several days of head-banging to get replies and posting via email working, lots of little things to get right both within Discourse and at your mail server, including mailer settings and firewall settings.
And I haven’t started to deal with mailer issues yet, specifically getting whitelisted at multiple ISPs. I do have SPF and DKIM working.