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 through another IONOS mailbox.


If a IONOS mailbox forwards an email, the 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 account@example.com:

    Return-Path: SRS0=OVdk=H5=example.com=m