I’m having issues sending emails from Nextcloud AIO. I have set up a Debian server with AIO running in Docker there, and on the host I have configured a Postfix server for mail delivery with SASL for relay authentication via submission with TLS.
I have verified that this configuration is correct by connecting to the mail server with openssl s_client and manually sending an email using PLAIN authentication (also successfully tested LOGIN) and the email successfully went through.
When configuring Nextcloud to use this server for email and sending a test email, I get the following error:
A problem occurred while sending the email. Please revise your settings. (Error: Email could not be sent. Check your mail server log)
I have enabled mail_smtpdebug = 1 in the nextcloud configuration, but nothing appears in the logs. I have also spun up a shell inside of the nextcloud container and run the same openssl s_client manual mail testing routine, and I was able to successfully send an email manually.