Michael Neurohr:
> Hi,
> the German Free Mailer GMX sends its newsletters with a Return-Path
> like "#16155...@gmx.net".
> I also recognized that sometimes the sending mail server does not
> provide a Return-Path at all.
The Return-Path: header is CREATED upon final delivery.
The Return-Path: header is IGNORED while receiving mail.
The value in the Return-Path: header is the envelope sender address
(see Wikipedia), i.e. the address in the MAIL FROM command during
an SMTP session.
If you use mail software that mis-handles addresses such as
"#16155...@gmx.net" then you need to use better mail software.
In the address local-part, the "#" character is not special according
to RFC 5321, the SMTP mail standard, or RFC 5322, the Internet
message standard.
Wietse
