SBS 2003, Exchange can't send emails to some recipients.
We couldn't send emails to a few recipients. I checked the queue and Exchange retries to send the emails for two days then send the 'Undeliverable' message from System Administrator back to the sending user.
firstname.lastname@example.org on 02/11/2008 14:41
The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
<server.business.local #4.0.0 smtp;421 Service Temporarily Unavailable>
I started with checking DNS and it quickly turned out that on our server machine after checking mx records for the recipients domains we have problems with (nslookup; set type=mx; domain.com) I get this:
DNS request timed out.
I looked up the DNS name (not mx record) for this particular domain and it worked OK. Then strangely enough the email for the recipient from this domain went through. When I tried nslookup; set type=mx; domain.com once again it worked OK.
I had done the same with another domain we had the same problem with and got it sorted out, but I would like to get to the bottom of the problem.