Operational Email - client doesn't want to receive

If you check their Black Listed checkbox in the Lead Info tab of their lead record, that will prevent them from receiving operational emails. You could also use a Change Data Value flow step in a smart campaign to do this if you want to do it for a number of leads at once.

You could create a segementation in your database for valid and invalid email addresses, and divide up the segments based on this criteria. It would allow you to have an idea of about how many people are unsubscribed vs. blacklisted vs. don't want operational emails. You could easily make a segment with a flow step like "if checks this box, then add to this segment" and then when you do the send, just avoid that segment. This would allow you to not 'taint' the number of blacklist vs actual unsubscribe.

Another option is to maintain a static list that you filter out in any smart campaign where you plan on sending this type of email.  I do this for the Customer Communication, which is sent operational to any Admin user, and there are some who do not wish to receive it.