[exim] stop myself from propagating invalid addresses

What is the best way for me to prevent myself from accidentally
sending mail to e.g.,
To: BIGLIST@???
Cc: OTHERLIST@??? <--forgot to add ".org"!
I want all addresses valid before any of them get a copy, lest I cause
responders also Cc'ing invalid addresses. I use smarthost, connecting
modem to run queue. Of course I only want to verify outgoing messages.