Can we resolve an Exchange problem using Plusnet

[#1]

We have a domain registered (and hosted) elsewhere and until today have been using dyn.com to direct our incoming mail to our Exchange server.  Outgoing email is sent out via Plusnet.
Our incoming email has stopped working, and this is we presume due to non-payment of the Dyn.com subscription.  However no-one has any idea of the login details for dyn.com account and from experience elsewhere reactivating the dyn.com account does not always reinstate the service correctly.
Can the MX record be changed to Plusnet without transferring the domain to allow delivery to out Exchange server by SMTP, or would we need to transfer the domain and change the A record to point back to the website?  Or move the website?
Or is there another solution?
TIA

[#2]

Hi there,
I can see from checking your account that you called in and spoke to one of our business support guys yesterday who advised you on this. Let me know if you still need any help.
Adam