More and more mail servers are starting to reject email if your outgoing mail server doesn’t have a reverse DNS record or PTR record. You can check if you have one by issuing the following command:

dig -x 127.0.0.1

Obviously, replace the 127.0.0.1 address with the relevant IP address of your mail server. You should receive a response like this:

; <<>> DiG 9.4.1-P1 <<>> -x 67.192.127.21
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17269
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;21.127.192.67.in-addr.arpa.	IN	PTR

;; ANSWER SECTION:
21.127.192.67.in-addr.arpa. 3600 IN	PTR	myserver.mydomain.com.

;; Query time: 217 msec
;; SERVER: 203.50.2.71#53(203.50.2.71)
;; WHEN: Thu Jul 10 11:09:58 2008
;; MSG SIZE  rcvd: 79

If you don't have one setup, you won't get a response in the ANSWER SECTION - so contact your host and ask them to set one up for you. It can help save a lot of headaches when you get bounce-back messages such as:

Could not deliver the message in the time limit specified.  Please
retry or contact your administrator.