Say someone has setup spf for his domain and sends an email to a user that has in roundcube enabled the sieve forward. If the message is forwarded without altering the message headers, this could result in a message being blocked or not relayed, because sending hosts ip, is not in the spf of the from: domain.
Possible solutions are: - add option if enabled, it replaces the From: with that of the email address of the sieve user. (Maybe move the original sender to the Reply-To header? Maybe exception for 'internal' forward?) - Upon processing the message, check the spf records, if they are enforced, do the above, otherwise do nothing. https://tools.ietf.org/html/rfc5228#section-4.2