Reverse DNS records point an ip address to a domain name or subdomain. They are also known as PTR records. This is in contrast to A or AAAA records, which point a domain name to an ip address.
Mail Servers
Misk.com rejects email from mail servers that do not have a proper reverse dns entry. Most email service providers (e.g., AOL, Comcast, Hotmail) also follow the same policy since it stops a significant amount of spam. Spammers generally don't have control over their ip space and its reverse dns records.
Check Reverse DNS
Use our DNS Lookup Tool to see if an ip address has its reverse dns setup correctly.
Setup Reverse DNS
Contact your ip provider (i.e., internet service provider) to create a reverse dns (PTR) record for that ip address. You'll need to give them the domain name or subdomain you'd like the ip address to point back to. Reverse dns records can only be created at the ip level and must be setup with the ip provider.
Forward Confirmed Reverse DNS (FCrDNS)
After you've created the reverse dns record, make sure the forward lookup (A or AAAA record) on the domain name maps back to the same ip address. For example, if you create a reverse dns record for the ip address 198.51.100.25 pointing to mail.example.com, you'll also need to make sure mail.example.com is pointing to 198.51.100.25. This is an important step since anyone can list any domain name for their ip's reverse dns record. By having the domain point back to that ip address, you've "confirmed" that the reverse dns record is valid. For more information, see Wikipedia.
Reverse DNS Similar to Dynamic / Default IP Space
Many ip providers setup a default reverse dns record for your ip address that looks like 203.0.113.93-static.example.com. Spammers generally don't have the ability to customize their reverse dns record. Hence, a default record is a sign of spam. Real mail servers use a custom reverse dns name (e.g., mta.example.com, smtp.example.com, etc.) that has been Forward Confirmed (FCrDNS). Not doing so can cause other mail servers to treat your email like spam or reject it.
Additional Information
Spamhaus - Why should I worry about reverse DNS (rDNS)?
Wikipedia - Reverse DNS Lookup
M3AAWG Email Sender Best Practices