Meine E-Mails (telekom.de) werden seit kurzem von Empfängern zurückgewiesen.
Fehlermeldung:

host gmail-smtp-in.l.google.com[...] said:

    550-5.7.26 This mail is unauthenticated, which poses a security risk to the

    550-5.7.26 sender and Gmail users, and has been blocked. The sender must

    550-5.7.26 authenticate with at least one of SPF or DKIM. For this message,

    550-5.7.26 DKIM checks did not pass and SPF check for [willker.de] did not

    pass 550-5.7.26 with ip: [...]. The sender should visit

    550-5.7.26  https://support.google.com/mail/answer/81126#authentication for

    550 5.7.26 instructions on setting up authentication.

    w1-20020a5d6081000000b003111f674aedsi2513795wrt.712 - gsmtp (in reply to

Was muss ich tun?

    end of DATA command)