SRS is rewriting an email's envelope sender so that it can be forwarded. Otherwise, the receiving mail server would use the sender's address (from the email's envelope), look up the corresponding SPF record from DNS, and find that the forwarding mail server does not match the legitimate mail servers listed in SPF.For example, email@example.com writes to firstname.lastname@example.org, which is forwarded to a Gmail account. Gmail will likely reject from that email from sumsi.at because the SPF record for example.org does not list sumsi.at.
SRS will replace the from address in the email's envelope (email@example.com) to SRS0...firstname.lastname@example.org. Note that the From: header in the email is not changed, so the recipient will not see any difference.
Setup for postfix is easy, just install the postsrsd Debian package.