Tracking/Auditing Donor Acknowledgement Letter

Hello! 

I'm curious if anyone has a system in place for tracking or doing an audit process of acknowledgement letters. As I'm sure you're aware, there are times when the volume of contributions is high which leads to a high volume of letters. We want to ensure that everyone receives their letter and tax receipt, so how can we make sure nothing falls through the cracks? We want to find a way to follow up and make sure everyone who needs a letter got one.

Any ideas? 

Thanks!