Meine Mails werden als SPAM deklariert

What is SPF? SPF is an extension to Internet email. It prevents unauthorized people from forging your email address. But for it to work, you may need to change some settings in your email program. Otherwise, the system may mistake you for an unauthorized person. If your mail was inadvertently blocked by a receiver who uses SPF, read on.
mail.absender.com rejected a message claiming to be from absender@domain.de.

mail.absender.com saw a message coming from the IP address 194.25.134.18 which is mailout04.sul.t-online.com; the sender claimed to be absender@domain.de.

However, domain.de has announced using SPF that it does not send mail out through 194.25.134.18. That is why the mail was rejected.
If you are absender@domain.de:

domain.de should have given you a way to send mail through an approved server.

If you are using a mail program instead of webmail, you may need to update the SMTP server configuration setting according to your ISP's instructions. You may also need to turn on authentication, and enter your username and password in your mail program's "Preferences".

If you run your own MTA, you may need to set a smarthost or relayhost. If you are mailing from outside your ISP's network, you may also need to make your MTA authenticate SMTP using SASL. Ideally your server should listen on port 587 as well as port 25.

If your mail was correctly sent, but was rejected because it passed through a forwarding service, you can either mail the final destination address directly (it should be shown in the bounce message) or you ask the forwarder to implement SRS. If neither of these suggestions is practical, change your "-all" to "?all" until a more comprehensive approach to sender authentication involving cryptography solves the forwarding problem for good. For more information on this problem, see pages 15-16 of the SPF Whitepaper.

You can also try emailing your recipient at an alternative email address.

Please contact your ISP for further assistance; ask them for help in configuring outbound SMTP email.

If your company needs further help, we provide a full range of consulting services to help you resolve these problems quickly.

    If you are confident your mail did go through an approved server:

    The system administrator for absender.de may have incorrectly configured its SPF record. This is a common cause of mistakes.

    Here's what you can do. Contact the system administrator responsible for absender.de and tell them that they need to change its SPF record so that it contains mailout04.sul.t-online.com. For example, they could change the record to something like

  v=spf1 a mx a:mailout04.sul.t-online.com -all

    If you can show this web page to your system administrator, they should be able to solve the problem.

If you did not send the message:

SPF successfully blocked a forgery attempt; someone tried to send mail pretending to be from you, but the message was rejected before anybody saw it. If you received a bounce message, you can delete it. This means SPF is working as designed.