We have recently changed our ISP, I have changed the mail and mailserver DNS 'A' records for our domain name to point to the new external static IP address assigned to the router by the new ISP (the MX record points to mail.<mydomain> as always) but I am not getting any email (though sending email works). Do I just have to wait will the change is reflected in DNS caches? I am slightly concerned because I can connect to the web email service made visible through the new router which suggests that the mail.<mydomain> static I.P. address change has happened.

mxtoolbox.com reports an 'A' type DNS record pointing to the static IP address of the router, but clicking on smtp diag on that web page results in :

Timeout occurred due to inactivity.

Port 25 is not open on mail.<mydomain> (presumably blocked by the router) should it be?

Have I missed something?

  • Where does the register think your name servers are? If you are self hosting dns, did you update them to know the ip addresses of where the dns servers are now? What is the result of whois example.com for your domain? – becomingwisest Jun 1 '12 at 21:32
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    How long has it been since the change? There's no such thing as DNS Propagation, it's a lie system administrations tell simple people who don't understand how DNS works to keep things simple. Does you new ISP block Port 25? Why didn't you keep both ISPs until you knew the new one was working? – Chris S Jun 1 '12 at 22:50
  • Can you hit port 25 from outside your network? Try mxtoolbox.com to ensure your DNS record is up to date and you can test your SMTP connection. – xeon Jun 2 '12 at 6:26
  • @becomingwisest A whois shews our domain to be hosted by our hosting company which is correct (this has not changed). The change happened about 14 hours ago. – Tim the Enchanter Jun 2 '12 at 6:48
  • @xeon mxtoolbox.com shews the A record for mail.<mydomain> to be pointing to the correct I.P. address. When I try the SMTP test it times out due to inactivity, what do I need to fix? – Tim the Enchanter Jun 2 '12 at 6:57
  1. Please don't use the word propagation when you should be using the words cache and/or caching. Using the word propagation in relation to DNS caching only serves to "propagate" the misunderstanding that DNS records propagate, which they don't.

  2. When referring to DNS caching, it's only relevant to systems that have those particular records cached, which most certainly is not very many in this case. I would find it very hard to believe that you're not recieving ANY email due to DNS record caching.

At any rate, it looks like the problem is a firewall issue, based on the fact that your tests seem to be indicating that you can't telnet to port 25 of your email server from the outside. Since you just changed your ISP and were allocated different ip addresses, I would suggest looking at the NAT configuration of your firewall to make sure you've got the correct external ip address NAT'ed to the correct internal ip address.

  • Point taken about "propagation", it turned out the problem was port 25 being blocked by the router, I was sure I had checked that before I left for the day, and of course I couldn't login remotely to check over the weekend. Thanks everyone. – Tim the Enchanter Jun 4 '12 at 9:55
  • Glad to help... – joeqwerty Jun 4 '12 at 13:10

Web email access doesn't matter, as xeon stated, you must be able to reach the MX server's IP on port 25 in order to receive mail. There are a lot of free tools to test the DNS configuration of your MX, A, and SPF/Text records for incoming and outgoing spam protection.

There is such a thing as "propagation" when properly updating SOA records in the DNS world, a record will hang around in another systems cache for as long as the minimum and TTL settings are on that DNS record.

Basically, if your TTL / Min timeout is set to a week, and you make a change, test it, then make another change, the DNS server you test on won't update again until the TTL expires. In most cases this is under an hour or up to a day. ( <3600 -> 86400 seconds)

This will depend on your DNS provider or configuration depending on if it's self managed or outsourced.

Hoope this helps

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