Folgende Fehlermeldung erhalte ich, wenn ich eine Mail beantworte, die ich von einem Gmail Konto erhalten habe:

This is the mail system at host mailout10.t-online.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

<xxxxx@xxxxx.com>: host ASPMX.L.GOOGLE.com[xx.xxx.xxx.xx] said:

    550-5.7.1 This message does not have authentication information or fails to

    pass 550-5.7.1 authentication checks. To best protect our users from spam,

    the 550-5.7.1 message has been blocked. Please visit 550-5.7.1

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

    5.7.1 information. l8si2009504wmg.93 - gsmtp (in reply to end of DATA

    command)

Leider habe ich auf dem angegebenen Link keine Lösung finden können.