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Out of the blue over the weekend, something strange has happened to my email server at work, as it suddenly stopped receiving emails. This has nothing to do with the server, but more to the fact that my mail.mydomain.com was being ignored.

Ping tests to mail.mydomain.com stopped working, although other.mydomain.com continued to work fine.

I've spent three hours on the phone to my ISP (BT) about it, and they couldn't help, except to find that it wasn't their fault.

As a workaround, I've used the 'other' A record (within my domain name DNS settings) to do what I need it to do (including adjusting my MX record accordingly), but despite deleting and re-adding the 'mail' A record, the mail.mydomain.com simply will not ping correctly, whereas other.mydomain.com will (both end up at the same machine on my network).

The TTL on the DNS settings for the domain name is 86400s, so maybe it's a waiting game, but having not changed anything on the domain name DNS settings for at least a year, it seems odd that such a thing would happen.

Any suggestions as to where I need to look to investigate further?

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If you told us what records were currently being published for your domain it might help –  symcbean Apr 24 '12 at 12:54
    
if you post the output of the ping mail.mydomain.com that would also be useful. –  Tom H Apr 26 '12 at 8:11
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May I urge you, and others who post DNS questions in particular, not to redact the domain in question. The DNS is designed to be exposed to public view anyway, and it makes troubleshooting these things much easier if you post the real domain in question. Your call, of course, but your solution will probably come quicker if you don't redact. –  MadHatter Apr 26 '12 at 8:41

2 Answers 2

up vote 1 down vote accepted

Its not clear whether you are on windows or linux, and for more specific help you might as well provide the domain details, its already a public record.

IN the meanwhile you can use some tools like this to check your Mx mail setup;
http://www.mxtoolbox.com/

"This test will list MX records for a domain in priority order. The MX lookup is done directly against the domain's authoritative name server, so changes to MX Records should show up instantly. You can click Diagnostics , which will connect to the mail server, verify reverse DNS records, perform a simple Open Relay check and measure response time performance. You may also check each MX record (IP Address) against 106 DNS based blacklists . (Commonly called RBLs, DNSBLs) "

the key tools you need are nslookup and dig for domain troubleshooting

for network troubleshooting its telnet, ping and tracert

You probably want to send the results of the lookup on the domain like so;

nslookup mail.mydomain.com

C:\working>nslookup mail.google.com
Server:  bebox.config
Address:  192.168.1.254

Non-authoritative answer:
Name:    googlemail.l.google.com
Addresses:  173.194.34.117, 173.194.34.118
Aliases:  mail.google.com

p.s. I would definitely change the TTL to something less;

;; QUESTION SECTION:
;mail.mydomain.com.             IN      A

;; ANSWER SECTION:
mail.mydomain.com.      1800    IN      A       192.58.31.64

Its also worth mentioning that ping and DNS resolution are kind of at different levels in the troubleshooting stack, basically if ping somedomain.com works, then you have proved the DNS resolution (unless you have /etc/hosts file) but it is not helpful to diagnose the underlying DNS issue, which you need whois, nslookup and/or dig or some web tools like the following;

http://looking-glass.taide.net/

https://www.ultratools.com/domainHealthReport

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Using mxtoolbox.com, I was able to find a whole raft of blacklistings that I wasn't even aware of. After lots of cleaning up and removing myself from these blacklists (some incurring costs!!) my server is now clean and (fingers crossed) working again, so thanks for your help: points awarded to you. –  Brett Rigby Jul 11 '12 at 12:09

Do mail.mydomain.com and other.mydomain.com point to the same machine? If so I guess you have two names for the same machine, right? If that is the case and your email functions as expected with other.mydomain.com and the necessary MX changes then this sounds as a DNS record corruption. Simply put, the DNS server ignores that record.

But not knowing how your ISP handles the domains and what DNS software is being used then this is just a guess.

I would certainly 1) remove completely the mail.mydomain.com A record and recreate it and/or 2) have your ISP clean your domain.com zone files entirely and restart them from scratch.

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Hmm. Yes, there was a couple of A records pointing to the same server, but this is usually the case when the server serves web pages and is a mail server too, as the www 'A' record also points at this box.... and always has done. Removing and re-creating the records didn't help, unfortunately. –  Brett Rigby Jul 11 '12 at 12:07

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