My darkmuse domain is currenly administered by Easyspace.  For quite a few years I've been using a separate nameserver to manage my DNS settings, but I recently changed (cancelled) my hosting and wanted to put everything back how it was initially.  Its taken a little while but support appear to have fixed it for me this afternoon.  For my own future reference, the MX record for easyspace should be:

@ 10 nicmail3.iomart.com.

For now, I'll be forwarding the domain to here (Posterous) with an A record.  

Fingers crossed