> What can I do to fix this? Could it be problems on the other side?

> Felix Natter gmx.net> writes:
>

>> What can I do to fix this? Could it be problems on the other side?

>
> For some reason this is a frequent place to trip up. I found that the
> fix is a simple
>
> (setq starttls-use-gnutls t
> starttls-gnutls-program "gnutls-cli"
> starttls-extra-arguments nil)

> 250 2.1.0 ... Sender ok
> 550 5.7.1 gmx.net>... Relaying denied. Proper authentication required.
> 221 2.0.0 ux-2s14.inf.fh-bonn-rhein-sieg.de closing connection
> smtpmail-send-it: Sending failed; SMTP protocol error
>
> gnutls-cli is installed.

> Felix Natter writes:
>

>> 250 2.1.0 ... Sender ok
>> 550 5.7.1 gmx.net>... Relaying denied. Proper authentication required.
>> 221 2.0.0 ux-2s14.inf.fh-bonn-rhein-sieg.de closing connection
>> smtpmail-send-it: Sending failed; SMTP protocol error
>>
>> gnutls-cli is installed.

>
> From the original mail, you set smtpmail-smtp-server, but did not set
> smtpmail-smtp-service. Since you're using the submission port 587 (as
> seen in your smtpmail-*-credentials) I think you're currently connecting
> to the SMTP on the regular port. Try setting smtpmail-smtp-service to
> 587.