We have Exchange 2007 sitting on our Server 2008 DC that receives and distributes E-Mail to all users on our network.
Our domain is registered with and our DNS record handled by DollarDNS and we use the backup MX record service provided by www.junkemailfilter.com
so that should or server go down for any reason e-mails are routed to them.
When a major client of sends us e-mails, every now and then the e-mail will be bounced back to the sender with the message 'all relevant MX records point to non-existent hosts', but then an e-mail from them will get through. As far as I can tell our DNS records seem to be ok, our mx record checks out ok and I am at a loss as to why these bounce backs are happening. Could anyone help with a reason as to why this is happening or give me some other things to check?