[exim-dev] EXPN and :fail:

In exim4 (4.34) I observe that when I set a custom error message using
:fail: in /etc/aliases (or using the fail command in an exim filter),
I see the custom message in the result of a VRFY command, and in the
result of a RCPT command (if the relevant ACL doesn't override the
message, or incorporates $acl_verify_message into its override) but I
don't see it in the result of an EXPN command, and it looks like there's
currently no way of configuring EXPN to show it. Is that by design?

For comparison, I observe that exim3 (3.35) shows the :fail: message for
all three commands, but shows a filter fail message only for EXPN, not
for VRFY or RCPT.

AMC