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                   The mail system

<...@gmail.com>: host gmail-smtp-in.l.google.com[173.194.76.26] 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. f5si5524230wmg.143 - gsmtp (in reply to end of DATA
    command)

Reporting-MTA: dns; mailout09.t-online.de
X-Postfix-Queue-ID: E10D5422C159
X-Postfix-Sender: rfc822; ...@....com
Arrival-Date: Mon, 25 Feb 2019 10:53:08 +0100 (CET)

Final-Recipient: rfc822; ...@gmail.com
Original-Recipient: rfc822;...@gmail.com
Action: failed
Status: 5.7.1
Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 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. f5si5524230wmg.143 - gsmtp