I have recently started to look after a mail system where users can host their email with a web mail front end. One of the features provided is a function to set up forwarding.
The problem I am facing, and which people before me have faced, is that when users forward mail to another mailbox, such as their own gmail address, Google sees the email as coming from an unauthorized IP address.
I understand that this is due to the sender domain have set up SPF records with a "-all" flag, and my IP address clearly does not belong there.
Gmail, and other providers, as far as I understand it, then cannot tell the difference between an actual forwarded message, and a spoofed spam message, and then subsequently greylists the address of my server.
I cannot help but think someone else must have solved this before.
The forwarding is done using exim, so what I am looking for is a guide, or even just the right keywords to use to search for, for an answer to this issue.