[Exim] Sender verification and address rewriting

As I understand it, exim verifies an envelope sender address by asking
whether a message (e.g. an error message) could successfully be sent to that
address. To determine this the address is processed as if it were an
envelope recipient address and considered valid if it is accepted by a
director or router.

However it seems that this principle does not extend to address rewriting.
For example we have a rewrite rule that applies to envelope recipients:

*@qmul.ac.uk $1@??? T

This is intended to help in the early stages of changing our internet domain
from qmw.ac.uk to qmul.ac.uk.
The idea is that incoming mail addressed to the new domain should be
delivered to the old addresses, while users here can configure their clients
to set their sender address to the new domain for outgoing mail without it
being rewritten by the central mail relays.

I had expected that this rule, because it applies to envelope recipients,
*would* be applied to sender addresses in the context of sender
verification. After all, mail can successfully be sent to the new addresses.
But exim doesn't seem to work that way - the rule is not applied and
verification fails.

Is this a problem with exim, or with my understanding?

Thanks, John.