Ok, if I specify

protocol=smtp

what does that mean: "Connect through SSL-TLS protocol" or "connect through STARTTLS protocol"?

That means connect using STARTTLS.

It seems to me that I ALWAYS have to specify "protocol=smtp" for ALL smtp-server connects.

You don't. If you are not using STARTTLS you don't specify "protocol=".

Is this done only by port number 465 vs 587?

No.

That would mean there is a hidden configuration by convention!

Indeed. That would be bad.

What if the webhoster would set up a special mail server at port 8888?

Then you would specify port 8888 in your "connect=" config