Hi Elena, we tried that to find a unique identifier (like what Jenn suggested), but since our SFDC team was doing the merges, it was hard to implement one (they're using a tool to do the merges).
Instead, I was able to pull a SFDC report to determine leads that had been merged. From there, I took those merged email addresses and ran a smart list to say "email address = XXX" and "was delivered email (is any) in the past (timeframe of merge)." Our biggest concerns were smart campaigns sending emails from the merges occurring (based upon Program Status changes). Luckily, we were in the clear, so this process was used more for spot-checking.
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