If you get an e-mail bounced back with a message something to the effect of no inverse DNS for 184.108.40.206, this was not our doing but something that went wrong at ARIN, the entity responsible for assignment of IP space and associated records. Re-send your e-mail and it should go through successfully now unless the receiving site is caching bad data.
I had a customer query me as to why the mail bounced, got this message, did an nslookup and indeed there was no inverse DNS for that, or any of our other servers.
I checked each of our name servers and they were all responding correctly. For some reason ARIN was failing to delegate inverse DNS requests for our IP space to our DNS servers. I went in to the section to update the records, made no changes what so ever, but after saving the non-changes it worked again.