I'm looking into using Zoho mail. I will want at least some mails forwarded to a third party server, which now applies SPF validation strictly. If the forwarded mail does not have its Return Path rewritten it will appear to have been sent from a server that is invalid for the original sender (as it now comes from Zoho). So I see that when you forward a mail the Return Path is rewritten so that the mail appears to come from my domain, not the original sender, and as I have no SPF record set there will be no validation failure. Fine.

But if the receiving server bounces the email (perhaps considered spam/malicious) it will come back to my mailbox at Zoho, and then will be forwarded straight on again to the other server that has just bounced it, so will bounce it again. This ends up in a loop. I see that you close the mailbox if there are multiple bounces which would be unhelpful. The proper way to rewrite is to use the SRS standard as at http://www.openspf.org/SRS. Will you be implementing this, if so when?