Probleme mit einigen Mails

[KleinesA]

Hallo,

einige Mails die ich sende kommen wieder zurück.

Quote:This is the mail system at host <mail.MeineDomain.de>

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system

<mail@empfaengerdomain.de>: host mail.empfaengerdomain.de[xxx.xxx.xxx.xxx] said: 551 This
mail server requires authentication before sending mail from a locally
hosted domain. Please reconfigure your mail client to authenticate before
sending mail. (in reply to MAIL FROM command)

Was muss ich hier noch tun?