[exim] Bad addresses in forwards

What is the best way to handle/verify valid forwarding addresses from users.

We’ve had a problem where old accounts with forwards in them just keep accepting email and then trying to deliver them to no longer existent remote email accounts.

Is there a way/best practice to verify the forwarding address before accepting the message being delivered? This would allow us to reject it at SMTP time rather than letting thousands of messages queue up for delivery and then ultimately bounce.

TIA

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Robert
inoc.net!rblayzor
XMPP: rblayzor.AT.inoc.net
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