Seit ca. 2 Monaten kommen E-Mails, die ich von T-Online verschicke nicht bei gmail-Adressen an. Die Fehlermeldung sieht so aus:

host gmail-smtp-in.l.google.com[108.177.15.26]
    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
    [t-online.de] did not 550-5.7.26 pass with ip: [5.9.161.126]. The sender
    should visit 550-5.7.26
    /support.google.com/mail/answer/81126#authentication" target="_blank" class="cmp_parsed_url" data-cmp_parsed_url="https://support.google.com/mail/answer/81126#authentication">https://support.google.com/mail/answer/81126#authentication">https://support.google.com/mail/answer/81126#authentication for 550 5.7.26
    instructions on setting up authentication.
    a11-20020a5d4d4b000000b0030e58857358si677899wru.208 - gsmtp (in reply to
    end of DATA command)

Wer kann da helfen?