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ClintonK
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Undeliverable Emails

I have recently moved an Exchange server from one ISP to a new one. The server configuration has not changed. Since moving to the new ISP, the majority of emails get bounced with an "Undeliverable" message. As of last night, the ISP say the Reverse DNS entry has been added but still they bounce back.
As the ISP is the only change made to the configuration I guess the problem must lie with them but I can't think what.
Suggestions welcome. Thanks
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ClintonK

8/22/2022 - Mon
Jaroslav Latal

Hello ClintonK,
do you use old ISP's SMTP server for sending email?

Regards,
Jarda
Jaroslav Latal

If it will not help, you chan check your SPF records.

Jarda
ClintonK

ASKER
I'm using Exchange so not using any ISP's smtp server
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Jaroslav Latal

You can use ISP's smtp server in Exchange. Depends on configuration.
Simon Butler (Sembee)

What does the NDR actually say? It will have more information on it than just undeliverable.
Are you referring to incoming or outgoing email?

Even if the PTR wasn't set i would expect a lot of email to be delivered.

Simon.
ClintonK

ASKER
Could not deliver the message in the time limit specified.  Please retry or contact your administrator.
            <mydomain.co.uk #4.4.7>
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Simon Butler (Sembee)

That is a timeout, doesn't help a great deal at all.
However if the problem was with the PTR then you would get something else.

Therefore you will have to look at the queues. That will tell you what the problem is.

Can you connect to remote servers on port 25?

telnet maila.microsoft.com 25

Are ALL email affected?
It could be that your ISP has blocked port 25 outbound, or there is a firewall issue that is blocking the traffic.

Simon.
ClintonK

ASKER
telnet maila.microsoft.com 25 returns:
"421 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel"

Yes, I think all email is affected
Simon Butler (Sembee)

Looks like Microsoft have shutdown that host, so not a good test.

Try Google instead:

telnet aspmx.l.google.com 25

Although the fact that you got the error would tend to suggest the port is open.

Need to know what the queues say really.

I would also check your firewall and any AV software installed to make sure they aren't blocking port 25 traffic in any way.

Simon.
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ClintonK

ASKER
I seem to have a number of SMTP queues in which there are emails with no sender and it's these that appear to be causing the problem. I've tried deleting them but the only option I get is "properties". I can delete items in the queue that have a sender but not those that don't.
If I send to an addresss that doesn't have a queue, then the email gets sent OK. If I send to an address that already has a queue then the email gets stuck behind those in the queue with no sender.
Simon Butler (Sembee)

Messages with no sender are either malformed or NDRs.
If you are unable to delete them then you may have a corrupt queue. That depends on the version of Exchange.
For example, with Exchange 2003, the queue is raw files and therefore the problems can come from badly configured AV.
With Exchange 2007 and higher the queue is actually a database and has to be deleted.

Simon.
ClintonK

ASKER
I'm running Exchange 2003 but there is nothing in the exchange queue folder.
C:\Program Files\Exchsrvr\Mailroot\vsi 1\Queue is empty and yet there are 9 queues shown in Exchange System Manager.
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Simon Butler (Sembee)

Those queues have items in them? Seeing queues in ESM is normal, they would usually be blank.

Simon.
ClintonK

ASKER
A little more information.
Not all emails through Exchange fail. I have just tried sending three emails to three Exchange servers and two out of the three get through. One server is Exchange 2003 and that works OK, the second is Exchange 2007 and that's get through OK. The one that fails and sits in the queue retrying is another Exchange 2003. I can't see why two will success and one will fail.
Simon Butler (Sembee)

Something blocking the traffic, restriction on the SMTP virtual server, something like that are the prime sources of the problem.

Simon.
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ClintonK

ASKER
I think I have DNS trouble. When I set my network card on the server to use itself only as DNS I can't access anything - can't even ping www.google.com
If I add the ISP's DNS address as a secondary DNS server then I can. Something must be wrong with my DNS as I shouldn't have to specify a secondary DNS I'm sure. I wonder whether this has something to do with the emails that can't find their way out of the server?
ASKER CERTIFIED SOLUTION
ClintonK

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ClintonK

ASKER
The change in the ISP and router address caused the problem when the forwarder was not manually updated.