Exchange 5.5 Host Unreachable

[hh]

Hi, this is a difficult one but also a good one to try and fathom out. I will try not to miss any details out.

Basically my company is using a Windows 2000 Server / Active Directory and Microsoft Exchange 5.5 setup to send and receive their e-mails. We are using a seperate windows 2000 server as a DNS server.

The problem is with certain e-mail addressing returning non delivery reports saying 'Host Unreachable'.

It was my job to check this out so I took a look at the setup of the Exchange server and saw that it is using Internet Mail Service (as opposed to X400 for example). When you send an e-mail to the e-mail addresses that are bouncing, the e-mails leave Outlook and a sent items mail but go to the IMS and sit in the Queue 'Outbound Messages awaiting delivery'. After 30 mins the message is still there but clicking on details reveals 'Destination Host Unreachable'. After two days the sender receives his undeliverable e-mail.

ON further investigation, there is a record in the DNS cache for the domain and when you ping the name it resolves an IP address although it does not reply but this is a firewall issue. I have tried sending e-mails to this address from my own e-mail account (which uses X400) and this is fine.

The Exchange server is using DNS to forward on e-mails.

The odd thing is that 99% of e-mails happily reach their destination but there is about 3 addresses/domains (and possibly ) more that are causing trouble.


Is anyone else having problems with the IMS connecter and can anybody help? I can give more information if needed.

Regards

 

[dennisbbb]

This is a DNS problem. Try changing the DNS server ip from another ISP on the Exchange server and see if that certain address gets through.

 

[hh]

This is really strange. For the past 10 months email has been fine but I haven't been able send anything over 1mb. Someone can send me a 5mb file but if I try to reply back with the file attached it times out and I get the 3010 host unreachable. There is no limit check in the exchange admin. and my ISP say's that it can take up to 10mb before it gets sent back.
I've tried pointing the exchange box directly towards the ISP's DNS but that doesn't seem to make a difference.

 

[hh]

Additionally, I can send the 5mb file to another mailbox on the same exchange server.

Jason

 

[Jeepix]

hh, who is your ISP?

 

[j93saturn]

Just ran into this issue a few days back with sending message attachments. I looked for a restriction on message size for a long time. Looked in Exchange, called ISP, searched web forums. Turns out my Spam filtering software which relays all messages from another server to my mail server was causing this. Other peoples problems were due to firewall (watchgaurd was the name mentioned) restrictions. Have you install anything on your network around the same time this started happening? Try to remove it if you did and see what happens.



James
MCP, MCSE

 

[hh]

Thanks for the response. I went through everything on the firewall. I finally broke down and called MS Exchange as I had reached the end of my rope. It turned out I had something they called a "Black hole router". For some reason my Linksys router wasn't sending out correct packets at the mtu of 1472. It was able to do it at 1400 and below even the the setting on the router was 1472. I could have killed those Linksys people. So I ended up buying a Belkin DSL router and everything worked immediately. Thanks for the contribution.

 

[ONotwen]

Check out article 198021 in the Microsoft Knowledge Base. Apparently IMS will cache DNS resolutions by default. This sounds like what is happening to you. The address you are trying to mail to (that is failing) probably had a recent change made to the mx entries in the zone file for the domain.

Unfortunately, I can't find out how to purge the cache or how long an entry stays alive in the cache. Your only option is to turn it off. If anyone figures this out please post it here. Hope this helps.

Thanks

 

[kilfarsnar]

I have been having a similar issue. I am running Exchange 5.5 on a Win NT4, SP6a server. The setup is such that all mail is routed to our Gateway (in a DMZ) unless it is destined for our remote office. In that case it is forwarded to a particular IP address which is the mail server in the other office. They are connected to us by static VPN. About once a day an email destined for the other office gets stuck in the Outbound queue with the error "Host Unreachable". Other email will go to the office at the same time. I can ping and browse the remote mail server. But these messages refuse to go! If the message is re-sent, it goes. What gives? I have offline defragged all databases, fully patched the server and Exchange, and even rebuilt the IMC! Any ideas?

 

[gp8]

Hi , I am experiencing the same problem. 99% of mails leave the IMS OK but a few will just sit and when looking at the details it says [Host unreachable]. I have tried nslookup to the host and get a valid response, I have even managed to ping the host in some instances I have tried different dns servers address's with no result. Any Ideas anyone. hh did you find the answer?

 

[kilfarsnar]

Gp8, does this happen with more than one domain? The problem I'm seeing only happens when I am sending to our other site; never with outbound email. Email is routed to the other exchange server by specifying the recipient host for that domain by it's IP address. So DNS does not seem to enter into it. The link is up and other email will be delivered promptly to the same site. What kind of configuration do you have?

 

[stonedog]

Hey guys, did this problem ever get a resolution? I'm having the same issues now - where mails get queued in the Outbound Awaiting Delivery queue - and some of the attachment sizes are also being misreported.

I'm running Exchange 5.5 SP4 on an NT4.0 SP6a server with Trend ScanMail. Mail routing software is Interscan.

 

[Benboy32]

Guys,

Have any of you found resolves for this as it is happening to me also...Mail stuck in outbound queue, host not reachable, the SMTP service is VERY slow to respond...we're using NT4 server, exchange 5.5 antigen and sophos.