Help configuring SMTP server

[lucrus73]

When it comes to SMTP servers, I have a few options, e.g. make UVDesk use one of my self hosted SMTP servers, or make it use Yahoo mail, or, again, use SendInBlue SMTP transactional email settings.
The problem is I tried all those choices and I managed to have it working only with one of my self hosted SMTP servers (which runs a old Courier-MTA 0.74 release), but I don't want to use that server, I only used that initially as temporary development solution.
The newer self hosted servers (three in total) that I tried but did not manage to have working, only log a connection and some SSL negotiation when UVDesk tries to send a message, but then they log a disconnect shortly after, before UVDesk even tries to complete the SMTP transaction. For example here is the log from Postfix on the same host.

UVDesk doesn't give any useful detail about what went wrong, except the red alert with "An error has occurred while sending email". What kind of error?

This happens both in dev and prod environments.

Using Yahoo and SendInBlue, UVDesk does not show any errors, not even the red alert, but it does not send any messages anyway. The real time log of SendInBlue shows nothing coming in. After testing those, I realized I did not clear the Symfony cache beforehand, so maybe that was the culprit in those cases.

There aren't any firewall rules on the UVDesk system. It is a Debian GNU/Linux with its own IP address. Every iptables chain is empty with ACCEPT policy.

It's not clear to me if I should specify SMTP settings in the Settings forms inside UVDesk, or into the .env file with the MAILER_DSN variable, or in the .env file with the MAILER_URL variable, or a combination of those three... and I don't know if/where UVDesk and/or Symfony log debug details about the SMTP connections.

My goal is to have UVDesk send emails using the locally installed SMTP server (Postfix).

Can you please help me understand and solve this problem?

[mateusvtoledo]

What's strange is that in the paid version of UVDesk this kind of thing doesn't happen. @webkul @uvdesk What can you tell us about this? Is there something we need to learn about?

I say this because the basis of a ticketing system is inbound and outbound emails.