I’m running a test setup (currently 2.4.0.beta4) at home on a local mini-PC with Ubuntu 16.04 LTS. Installed it using the excellent 30 minutes install guide. It does have a FQDN. Sending out email via a mailbox at my ISP with SMTP on port 587 with plain authentication has been working fine for over a year.
I just realized that I did not get any email notifications for some time like “new version available”. Checking Admin > Emails > Skipped shows that everything gets a
550-Bad HELO: localhost.localdomain does not exist - Please see RFC 2821
Looking back in the mailbox this could have been started with 2.4.0.beta2. But it could also be some policy change at my ISP at about the same time (end July 2019). Not sure where to start. Where does this
localhost.localdomain come from? During installation I only had to edit app.yml and this shows my FQDN correctly at DISCOURSE_HOSTNAME: