`reject_invalid_helo_hostname` may be of some use **but `reject_unknown_helo_hostname` is a huge risk for rejecting legitimate messages**. For example even spamcop.net confirmation emails are rejected due to their servers configuration. If, by any chance, the helo name does not match a valid IP address then a legitimate message is very well rejected. Last week I had this issue even with Apple Cloud - legitimate messages beeing rejected! **I advise change `reject_unknown_helo_hostname` to ` warn_if_reject reject_unknown_helo_hostname` then watch the logs for some time see the results.** **Please don't jump right in and operate changes without rigorous testing. Or it may affect all of us without even knowing it.**