[imp] Default Identity for multiple mail domains

[kenneth yip]

Please excuse me if the same problem has been solved in the past
already.

I am using a single IMP server to serve multiple mail domains. A user
may have account on 1 or more mail domains and is allowed to
choose which mail domain to login from a select list, say domain1
and domain2.

In IMP 3.x, when user compose e-mail, the default sender address
is username at log.in.domain, i.e. if the user logged in to domain1,
the sender address will become user at domain1 and if the user logged
in to domain2, the sender address will become user at domain2.

In IMP4.0.2, it seems that the user's default mail domain is stored
in the backend DB when the user login IMP for the first time. So,
when the user login domain1 the first time the user login IMP, the
default sender address will become user at domain1 no matter
which domain the user subsequently log on to.

Is there any way I can change this new behavior on IMP 4.0.2 back
to IMP 3.x, i.e. the default sender is generated based on which
domain that the user has logged on to?

I tried looking into hooks.php, but couldn't locate something helpful
in my case.

rgds,
ky