[Exim] Retry strategies for non-MX hosts?

Quite a number of places which we deal with have, in recent years,
phased out host-based email domains, without supplying any MX records
for the old hosts; these old host-based addresses tend to re-emerge
out of the woodwork for one reason or another, and our folks try
sending mails to them, which then hang around pointlessly retrying for
days on end trying to contact a non-existent SMTP server on the host
corresponding to that A record.

Where we've spotted these dead domains, we can put an entry in a local
blocklist declaring this domain to be unreachable, so that our users
get an immediate rejection (and anyone trying to send mail to us
purporting to be from these dead domains will also get rejected, but
thereby hangs an unrelated tale...)

Anyhow, coming back to our retry strategy, for any of these dead
domains whose names we haven't yet entered into the relevant blocking
list here...

Some of these dead "domains" (I mean the IP from the corresponding A
record) will explicitly reject an SMTP connection attempt, while
others just sit around ignoring the call until it times out.

While reading the documentation recently (section 32.3 refers), I
spotted the refused_A failure reason (and the timeout_connect_A in
recent exim versions). These are presumably intended for just the
kind of circumstances I'm describing; but a search of exim-users and
the FAQ didn't find me any discussions of the practical implications.

Seems to me we could adopt a fairly laid-back retry strategy for
errors where no MX record is involved, with less-frequent retries, and
an earlier final failure than we'd otherwise use? Have folks tried
this - any recommendations please?

thanks