Email Header: "Return-Path" Contains Unknown Address

Please Note:

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If you do not know the email address entered under Return-Path in the email header, this email may have been forwarded to your address via another 1&1 IONOS mailbox.

If a 1&1 IONOS mailbox forwards an email, the 1&1 IONOS mail system stores a coded entry in "Return-Path" according to the so-called Sender Rewriting Scheme(SRS)
:

  • The SRS ensures that the original sender of a forwarded email remains recognizable by an entry in the email header. Undeliverable messages can be sent to the original station.
  • The entry is created according to the following pattern: SRS0=HHH=TTT=hostname=local-part@domain, where HHH is a hash and TT is a timestamp;

    in the following example the email was originally sent from max.mustermann@gmail.com:

    Return-Path: SRS0=OVdk=H5=gmail.com=m
    ax.mustermann@srs2.kundenserver.de