Updated by Gerhard Kleewein
The primary reason for setting up an SPF record is to prevent SPAM from being sent from a domain name. An SPF record defines which IP addresses shall be allowed to send email for a domain name and how receiving mail servers must treat emails originating from non-permitted IP addresses.
SPF reaches its limits when forwarding external e-mails.
The fundamental problem: The forwarding email server is very likely not authorized to send emails from the sender domain due to the SPF record (forwarding/redirecting is one form of sending emails).
To eliminate this fact, EDIS implemented an (experimental) technology called SRS https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme.
SRS rewrites the "envelope sender" and each email is provided with a cryptographic "hash" so that forwarding against the SPF configuration is possible.
5.0.0 smtp; 550 rejecting for sender policy framework