I have a brand new Nextcloud AIO install, only a few days old as of this post. So far I have everything working except the SMTP link to my corporate mail server. I also have an existing, much older nextcloud instance on the same subnet that is working fine.
My mail server (Zimbra) is on the same internal subnet as both my nextcloud instances. No firewalls or routing between the any of them. The new nextcloud instance (29.0.4) refused to link up to be able to send mail. I can watch the log on the zimbra server and see nextcloud connect successfully, Zimbra reports the IP as allowlisted for forwarding since they are on the same subnet, but Nextcloud refuses to move on to the next step and actually deliver the test email. It is configured as SMTP with no authentication and using the server IP address and port 25. When I click the send email button to test the config a red error pops up and says “A problem occurred while sending the email. Please revise your settings. (Error: Email could not be sent. Check your mail server log)”
I can verify the box has access because I can log into the debian installation that is running the nextcloud AIO instance and send test emails using telnet with no authentication.
I do not know what to do next.