I must apologise for the e-mail problems. Until yesterday we used a company called XCalibre (now owned by Webfusion) to host our e-mail. It has been like that for some years without a hitch. The only issue we have had with them is payment - they are quite a difficult company to pay.
A couple of weeks ago we received their annual invoice. We tried to pay them yesterday but their systems wouldn't take our (HSBC) sort code. When we asked them about it they disabled our account - hence the bounced emails! When I contacted tech support about it (not realising they had cancelled our account as they didn't tell us) they disabled my support account. What a bizarre way to carry on.
Needless to say our e-mail is hosted somewhere else now and hopefully the DNS implications have worked themselves out overnight.