To close the SASL mail delivery thread, here is what happened:

 I found out that

  - The way I need to my mailserver to authenticate at the upstream provider
    required SASL, which would be available for postfix and sendmail, but
    not in the version in the base system (both require cyrus-sasl).

  - The way my upstream provider implements authentication is completely
    useless to me. I have complained to their tech-support and will stop
    using this service.

So I actually did not get arround installing either sendmail or postfix with
the necessary patches, and can't give a summary or comparision.

Sorry,

Martin