On Fri, Oct 20, 2017 at 07:01:04AM +0200, Ben Stover wrote:

In stunnels *.conf file I can specify the mail server name and the port for a connection.

How can I tell stunnel to use STARTTLS as connect type? How can I tell stunnel to use SSL/TLS as connect type?

Does stunnel support both connect types at all or only one?

Is the connect type for SMTP completely determined by port e.g.:

STARTTLS over port 587 and SSL/TLS over port 465 or can both connect types use both ports?

Take a look at the excellent stunnel documentation (hint: "protocol").

Assume I do not get the supported connect type information from my webhoster:

This is highly unlikely; how else are you supposed to configure your e-mail client? (assuming you are trying to setup a tunnel to your provider's e-mail server)

How can I find it out from my (client) side?

Still, there is a way: if you have somehow managed to guess the correct hostname and port to connect to, you can use "nc -v" or "telnet". If the port is 25, or if the server greets you with an SMTP banner (usually "220 something" or "220-something"), then this is a cleartext connection and you want to set up STARTTLS (you can test for it by issuing a couple of SMTP commands and then trying STARTTLS). If the server just hangs there and waits for you to send something, then it's probably a TLS connection; you can test for that by running:

openssl s_client -connect hostname:port

Still, I do indeed find it highly unlikely that your hosting provider will not have given you enough information.

What are there error messages?

Not sure what this question refers to.

G'luck, Peter