Lets say I have my mail server server mail for domain foo.com. There is a thirdparty service that emails message and look like they are coming from the email address firstname.lastname@example.org. This thirdparty domain is bar.com.
Now we want bar.com to spoof our email@example.com so when others reply we get the email and bar.com does not. However, we have noticed that when bar.com emails a message from firstname.lastname@example.org to any other address in foo.com that sometimes(not always) it never shows up. There is no bounce message or rejection in the maillogs.
We think it might be hitting the spam filter and never being delievered. How do we whitelist bar.com as the sending domain? email@example.com is already whitelisted but how do we tell our mail server that it is OK for bar.com to be sending mail for firstname.lastname@example.org