uemurad -> RE: There was a SMTP communication problem with the recipient's email srvr (14.Sep.2009 10:44:52 AM)
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PTR and A records are types of DNS entries. An A-record associates a Fully Qualified Domain Name (FQDN) to an IP address. For Email, it needs to be a publicly routable address (not a private address). An MX-record associates a priority value (0-255) with a FQDN which must be defined by an A-record. A PTR-record is used for reverse-lookups, associating an IP address with a FQDN. The difference between A and PTR records is how they are queried. When your system asks the public DNS systems to tell you the IP address of a FQDN, the A-record is consulted. When your system asks the public DNS systems to tell you the FQDN of an IP address, the PTR-record is consulted. Hotmail (and other Email hosts) try to filter out spam by executing a reverse DNS lookup for the IP address from which mail is coming. If you have no PTR record, the query will come back null, and your message will be rejected. Similarly, if the domain in the FQDN returned does not match the domain in the sending address (yourname@yourdomain.com), it can also be rejected or tagged as spam.
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