Thank you both for your patience I really appreciate it.
From memory, which may be incomplete

1) change config on router to disconnect the mail server from Internet.
2) run SME backup to windows share on my win 10 workstation.
3) on completion, disconnect server from network. rebuild from DVD.
4) configure server as previously i.e. same name, IP address, domain.
5) run yum update from console
6) logon from web interface. set remote access, windows domain name and domain controller.
7) Configure workstation backup as before.

try restore - fails due to permissions.
9) move workstation of of domain to workgroup, reboot
10) login windows as local admin, rejoin domain, reboot
11) login to windows domain
12) attempt restore from web page - success.
13) reconfig router for email link.
14) check email

I think that covers it............