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We use iis 6 to send out emails for our website but some email addresses bouce backs. It only appears to be 2 addresses. There is at least 100 different email addresses sent to at the same time as these. Here are the 2 different messages we get.

First one.

You do not have permission to send to this recipient. For assistance, contact your system administrator. secure.example.com #5.7.1 smtp;550 5.7.1 Fix reverse DNS for xx.xx.xxx.xxx or use your ISP's mail server

Second one.

Could not deliver the message in the time limit specified. Please retry or contact your administrator. secure.example.com #4.4.7

Not sure if they both relate to reverse dns. If you can help or require more information please let me know.

marked as duplicate by Andrew B, mdpc, Cristian Ciupitu, Mathias R. Jessen, Michael Hampton Jul 19 '14 at 5:34

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.


Check if the IP address you are using for outgoing SMTP has a DNS PTR record ("Pointer Resource Record"). Not every IP address has a corresponding PTR record, despite the fact that it's recommended in RFC 19212.

You could check the PTR record using an online service, like DNSReport.com. There is a box "IP Information" on the first page.

The second case you mentioned seems to be a temporary problem, probably of the receiving SMTP server.

  • Have got the ip information. Here is is IP address: Reverse DNS: [No reverse DNS entry per ns.rackspace.com.] Reverse DNS authenticity: [Unknown] ASN: 15395 ASN Name: UNSPECIFIED (UK Rackspace) IP range connectivity: 1 Registrar (per ASN): RIPE Country (per IP registrar): GB [United Kingdom] Country Currency: GBP [United Kingdom Pounds] Country IP Range: to Country fraud profile: Normal – user7468 Jun 23 '09 at 10:00
  • City (per outside source): Unknown Country (per outside source): UK [United Kingdom] Private (internal) IP? No IP address registrar: whois.arin.net Known Proxy? No Link for WHOIS: – user7468 Jun 23 '09 at 10:00
  • the ip address in the messages relate to example.com but we have a different ip address for secure.example.com. Does a PTR want putting on the same ip address of example.com or scure.example.com – user7468 Jun 23 '09 at 10:14
  • You should ask your provider (Rackspace) to insert a PTR record for the IP address. If it's not possible for them to insert your own domain (e.g. yourmailerserver.example.com), change the FQDN on your default SMTP server to match the corresponding A and PTR records they created. – splattne Jun 23 '09 at 10:25
  • Will get them to create one and will let you know the result. – user7468 Jun 23 '09 at 10:32

Firstly, your mail server MUST say HELO with it's canonical host name, that is the same value as the hostname command returns when run on the command line. Under postfix this is the value of the myhostname variable in /etc/postfix/main.cf.

Secondly, the forward and reverse pointers for your server in DNS must match, and must match the hostname value above. For example

[dave@odessa ~]$ host odessa.cheney.net
odessa.cheney.net has address
[dave@odessa ~]$ host domain name pointer odessa.cheney.net.

Thirdly, check that your mail server is not acting as a open relay. If your mail server is configured to relay mail from other hosts, you should be very careful to ensure that those hosts themselves are not open relays, or are properly protected from accepting messages from unauthenticated hosts There are plenty of sites out there that will make a check for you, http://www.checkor.com/ for example.

Lastly, with the large sites, some of our mail will marked as spam, it's just the law of averages. It's your responsibility to make sure you comply with the appropriate regulations for send automated or bulk email, like CAN-SPAM. The most important part of this is a straight forward way for people to unsubscribe from your mailing lists. If you make this hard for people then you're users are more likely to hit the large spam button in their mail client which will be fed back to the mail host and will work against your server.


In an effort to curb incoming spam, many mail servers these days do a "reverse DNS lookup". That is, they take a look at the IP address connecting to them and then try and do a reverse DNS lookup to see if there is ANY response.

This prevents any random IP address from becoming a mail server and sending out thousands and thousands of emails.

Some mail servers even go an extra step and have additional points/verification based on WHAT the reverse DNS came back with. For the most part, if you have any sort of reverse DNS set up for your IP you'll be "okay".

If you don't have control over the reverse DNS for your IP address contact your ISP and ask them to create a reverse DNS record (ie. PTR record) for the IP address that is sending out your email.

Alternatively, you could opt to use a mail server (which has a reverse DNS record already in place) or you could use your ISP's mail server and relay your mail through them.

Hope this helps.


Only the first message seems to have any relationship with "Reverse DNS". But, since you edited it to remove the important information (the IP address), it is hard to say more.


As the first message is clearly a DNS reverse error, i think the second may be a consequance of the reverse DNS error, the connection attempted was given a time penality by a smtp antispam proxy or the remote smtp server was simply offline.