I'm having problems solving an email issue for a client.

They run a lunchtime food business - people email their orders for sandwiches etc to them, they deliver (in a business park).

When someone sends an order, they get an auto-response. For the last week or so a number of the auto-responses have been bouncing back to them with 'unrouteable mail domain' errors. They also get these errors when they try and email the clients directly.

I've checked the domains via www.dnsstuff.com and they all seem fine. I've also sent mail to the domains (from the same server as the client) and have had no problems).

The client's DNS and MX records look fine, so I'm just a bit stumped as to what the problem might be?

Any advice appreciated.