sending mail willnot work

This is the error message i receive:
(translated from Dutch)

Cann't send the e-mail because one of the addressed is refused by the server.
The refused address is xxxxxxxxxxxxxxxx
Subject 'test', Account: 'mail.xxxxxxxxxxxxx.net', Server: 'mail.rs-xxxxxxxxxxxx.net', Protocol: SMTP, Reaction from server: '503 This mail server requires authentication. Please check your mail client settings.', Poort: 25, Beveiligd(SSL): Nee, Serverfout: 503, Foutnummer: 0x800CCC79


I have entered my authenticaton correct as
user@domain.ext and password. Also in Outlook i entered the authentication is needed for sending mail.

Could anyone please advise ?

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I think I found the problem:
In the Mailsettings from the server i can't DISABLE the SMTP setting. SMTP when i use Relaying - Authorisatoin required -

When i use Relaying open, erverything works. So it is an authentication problem