[exim] Outbound SMTP interface selection vs SMTP connection …

We have an SMTP transport which selects the outbound interface based on
a number of factors.

We have noticed that if a mail is not delivered immediately, it is
sometimes delivered as an additional message on an existing connection
which may not be using the same interface.  In this case, the logs show
it being delivered as the result of an "exim -MC" command.

Is this expected behaviour, and is there anyway to control it so that
the interface selection is always honoured for a message?

Paul