PTR record is a piece of information based on which the DNS realises IP address ---> name translation. PTR records are not recorded in the same zone as the others (e.g. A or CNAME records) but in their own zone, the name of which starts with a seemingly strange word identifier (including something like addr arpa etc.) and ends with the reverse order of IP network octets. The problem with mail is the purpose PTR records are sometimes used for: if the client requires a service on the Internet, the inquiry is sent and the target server tries to perform a reverse translation based on the sender's IP -- if the PTR record and its zone exist and are "visible", the action results in translation to your domain name, thus confirming your existence. If the translation fails, some servers will deny their services.
