undelivered emails to certain domains

We have setup an SBS server 2003 that is acting as mail server (it is responsible for sending and receiving emails directly) with an static IP address. In 90% of the times the emails are reaching the recipients but there are some domains for which when an email is being sent from this server does not reach the recipient. However, we are able to send an email to the same recipient through gmail or another non sbs server. We have contacted the ISP who is hosting our recipient's email to check whether our IP is blacklisted on their server and the response was negative. Any idea why this is happening? is our IP address tagged incorrectly?
yourikhAsked:
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abdulzisCommented:
Do you have reverse DNS and SPF record setup correctly?
What does the queue status say?
Try to drop an email through telnet to that domain and see what happens.
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Dimarc67Commented:
I've come across this before on more than one occasion.  Forgive me if I explain anything you already know or have already addressed.

As an anti-spam precaution, many ISP's will block e-mails from any IP address that does not have a DNS PTR record (a reverse-lookup record) associated with it.  The result is exactly what you describe.

If you haven't already, you'll need to contact your ISP and request that a PTR record be created for your mail server's IP address.  It's also best to use the same name as your MX record for the PTR.  

For instance:
DNS A record:  mymailserver.mydomain.com..........120.121.123.124
DNS MX record:   [10] mymailserver.mydomain.com
DNS PTR record (on ISP's host):  120.121.123.124...........mymailserver.mydomain.com
(Actually, reverse lookup records are usually represented backward. So the above PTR is correctly displayed as "124.123.121.120.in-addr.arpa", but you'll rarely need to provide it that way.)

Hope this helps.

Dimarc67
New York, NY
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yourikhAuthor Commented:
Thanks for the reply. I'll try to contact the ISP to see if the PTR can be created. I had a case where I asked the ISP (Bell Canada) to create the PTR and they said they do not provide that service. Is there any workarounds in case this other ISP (Rogers) tells me the same thing?
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abdulzisCommented:
Any DNS/ISP should have the facility to create a reverse DNS/PTR record as it is a common thing.
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yourikhAuthor Commented:
I'll keep you guys posted once I make the call to the ISP.
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Dimarc67Commented:
In fact, almost by definition, ONLY the ISP can create the PTR since the IP is presumably issued to them by ICANN.  There are some services that provide PTR ability outside the ISP, but it still must be supported by the ISP.

Unfortunately, it is possible that an ISP would refuse to create the record if your SLA does not allow connecting servers to the provided internet connection.
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yourikhAuthor Commented:
Hi guys,

thaks for the help. It took Rogers 10 days to setup the DNS reverse lookup but now everything seems to be working fine.
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