Support disabling automatic use of TLS

⚠️ This issue respects the following points: ⚠️ • This is a bug, not a question or a configuration/webserver/proxy issue. • This issue is not already reported on [Github]( OR [Nextcloud Community Forum]( _(I've searched it)_. • Nextcloud Server is up to date. See [Maintenance and Release Schedule]( for supported versions. • I agree to follow Nextcloud's [Code of Conduct]( Bug description Hello, I came upon an issue with our Exchange send connector, that allows anonymous relay from some of our internal IP addresses. Our connector advertises TLS but has no TLS Certificate bound to it (which is not a good thing), but in the past Nextcloud did not care and sent emails with no TLS using this same connector. Recently, it seems that TLS is enforced by Symfony Mailer if STARTTLS is advertised at EHLO. And thus emails are not sent anymore, because of the aforementioned settings (or lack thereof) in our Exchange. It worked perfectly in the past, so I don…