Bypass lists can contain other types of entries other than IP. Using an IP to bypass is fastest because it avoids DNS lookups. If there are any hostnames or domains in the bypass list, then a reverse lookup has to be done on the IP of every incoming connection in order to evaluate the PTR record against the bypass list. The additional DNS lookups will have some small performance impact.

However, if you would like to bypass specific senders and domains, all you need to do is add them to the greylist bypass file. The format is one per line, just as you would do when adding an IP.