We are developing a SaaS website application that lets users send invoices to their clients. Ideally, these e-mails should appear to be originating from our customers, so the sender e-mail address domain will not match the reverse IP entry for our server. In effect we would be forging their e-mail address, but of course with their consent.
Will that result in a higher probability of being marked as a spammer / their e-mails being marked as spam?
If yes, how bad is the penalty?
And what about people who have an e-mail address originating form an SPF-enabled domain? I guess it should be the majority of the big e-mail providers.