The 'right to track' here is because Mozilla is getting blacklisted for sending to now-nonexistent addresses, and when they're blacklisted, it's hard to get off the blacklist.

As an example, one company I work for is on the blacklist RFC-Clueless, which blacklists you for not following the RFC and having the required default mailboxes (webmaster@, abuse@, etc). We use gmail, and it was a screw-up in their system - something I got them to fix about 2.5 years ago. Despite multiple requests to get off the RFC-Clueless list (their request link being obfuscated), they've done nothing about it. It's particularly galling that a blacklist that blocks you for 'being a bad netizen' is themselves an atrocious netizen.

So, if a client has an IT dept that subscribes to RFC-Clueless, there's simply no way we can send them email - it's very difficult for our sales staff to convince their contacts to start that conversation with their own IT, especially in the initial stages of the trial.

So I can understand that Mozilla might want to keep themselves off mailing blacklists, especially when they're sending mail to now-dead addresses that have previously asked for mail to be sent.