How best to map users to domain name for login

[yani at ecoco.co.uk]

radiusd -v
radiusd: FreeRADIUS Version 3.0.4, for host x86_64-redhat-linux-gnu, 
built on Jan 17 2017 at 18:49:55

Hi.
I've been reading the manuals to attempt to determine the best way to
configure free radius for my requirements but can't decide which is the
best route to follow.

I want to be able to authenticate users to my email service and manage
them according to the domain they belong to  so fred at domaina.com is not
the same as fred at domainb.com.  But in both cases the first part is the
login user name within the domain.

I have configured all of the systems to work with the default radius
configuration  (Exim MTA   Cyrus sasl  and ssh logins working fine) and
the mail server is correctly operating with virtual email domains (i.e
fred at domaina.com is not the same as fred at domainb.com).

I have looked at freeradius virtual servers - and have  considered
using free radius realms dont see how either are actually the way forward -  It seems
that virtual servers will need a database system creating for every
instance( am i actually correct here) and that realms are really for
forwarding requests to other free radius servers - when all i need at
the moment is a single server handling multiple domain based login
groups.

I understand from the documentation that  I can create a local realm
like this

realm domaina.com {
     type= radius
     authhost= LOCAL
     accthost= LOCAL

I suspect I'm on the right track here - but haven't figured out how to
create users in the database/system  that reflect this

Please advise on the most appropriate way of configuring
freeradius to achieve logins for multiple internet domains.

  Yani