Our Company is developing an email marketing cum brand promotion application; this application will be using the our sendmail server for sending out bulk emails. We have already made our mailserver spf, dkim and domain-key compliant; we also have valid reverse dns records records for it. Now our concern is our application will be designed such that our registered clients will be able to send out mass email campaigns using their own email ids - say if our domain is promote.com and one of our clients are having their domain as abc.com then our clients will be sending out emails using the source address email@example.com and the mail will be send out using the mail server for promotions.com
Now my question is: what needs to be done at my end so that all the recipient email servers can authenticate that mail.promotions.com as permitted sender for domain abc.com ? How do I comply with reverse dns checks, spf, dkim and domain-key data in this case?