TLS issue / STARTTLS support

[busslina]

I commented this issue here.

Thanks

Update I:

I think the issue can be related to STARTTLS. If I'm correct, this protocol starts in plain text and then it upgrades to TLS after negotiating it. So this library attempts to stablish a TLS connection when ssl is true. So seems that is not possible to connect to a Postfix server (at least I didn't found how to disable STARTTLS).

Update II:

I thought that Dart didn't support STARTTLS but it seems it can be done this way:
https://stackoverflow.com/a/17643296/4681977
https://api.dart.dev/main/627cd3047d2b52e23ba0cd3e31782c0f5ac97e85/dart-io/SecureSocket/secure.html

[close2]

I've added some more information in dart-lang/sdk#52886

But the TL;DR is: Don't use ssl. mailer will automatically upgrade the connection to an encrypted connection using the smtp command starttls. If the server does not support tls mailer will either continue over an insecure connection if the option allowInsecure is given (off by default) or fail otherwise.

There are very few ssl smtp servers. Usually smtp connections are started over insecure connections and then upgraded to secure ones, using the starttls command.

[busslina]

Cool, thanks. This issue pushed me into studying how SMTP works.

There are very few ssl smtp servers.

My server is one of them and it works great with ssl to true.

So now I fully understand how this package works. Maybe would be a good idea to specify this info on the SmtpServer class documentation.

And thank you again :)