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My client's network is connected to the internet via T1. Their system has been working fine for months. Now they call to say that, all of a sudden, they can no longer send email or access the internet. But strangely enough, they are still receiving email. Email is hosted at their ISP. What could be causing this problem? Please know that pinging an external, public IP address works.

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So it sounds like DNS is broke, especially if you can ping external IP but not ping for example…check with the ISP to make sure that the DNS hasn’t changed on their end, see if you can ping the dns server itself from one of the machines, you probably wont be able to, i dont know you clients setup as far as the router goes or what kind of router they have, but sounds like the router or server is handing out the wrong DNS or it has changed by the ISP and no one changed it on there end…also you could send email by changed the mail server to its IP and not name, example incoming mail server: change that to incoming mail server: x.x.x.x then there email would at least work

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  • Bobkberg
    jpagel makes a good point - bring up a command prompt and enter "ping", and see if you get name resolution, followed by the ping. You didn't specify whether pinging an external IP address involved DNS resolution or not. Next question is outbound mail. Does your outbound email server also belong to the ISP, or are they just providing your local email. What I mean to say is: Are you using an outbound email server which is NOT part of your ISP. Many ISPs these days are blocking port 25 outbound. Also - check to see if ALL systems are affected by this. There are viruses, spyware and such which install local proxy software which selectively block things. Use HijackThis to check on that. Have you (or them) called their ISP to see if anything has changed? (Like the IP address of the outbound mail server, or someone installing a proxy server? Have you gone on-site to check things out yourself? Write back with more info to the group so that we can track this. Bob
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  • Jpagel
    and btw, bob good point on the ISP blocking port 25 to fight "spam" even though the spammers are just going to change ports...but hey who knows... the next best secure email port is 587 (i know a lot of isp's are switching to that)
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  • Delebute2004
    All the prior suggestions are very good and are the best ways to troubleshoot your current issue. if you can receive mail then you definitely have a connection to your ISP; the issue with sending i have seen that many ISP's block port 25; which makes you have to use their smtp server to send out. if you have a good relationship with this ISP, you should have them setup reverse DNS for your mail server and try to check your AD DNS setup as well. Are you using the any external DNS servers for your domain or using your DNS server and the ISP as forwarders for anything outside your domain? Lot's of stuff to review and check, any changes on your Firewall or Router configurations?
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  • Juscelino
    I had a very similar issue where I could receive e-mails, however, I could not send any e-mails and it turned out to be that my ISP (SBC Yahoo!) was blocking port 25. You may want to look into port 25 being blocked since, just like your client, everything was working fine, then it happened all of a sudden and I figured that out after attempting everything that I could think of and losing a couple of days worth of messages.
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  • Ciscocat6k
    Can you telnet to your ISP on port 25? This would be a quick check to see if it is blocked. If they are not allowing a telnet connection then you would get some message stating that connection is refused, but if all you get is a time out then you probably are blocked. Just my .02 worth CiscoCat6k - CCDP
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