The email queue doesn't queue blocked messages, so you don't have to worry about any past emails being "released." However, any scheduled jobs, scheduled reports/dashboards, Time Based Workflow Rules, Time Delay Process Builder actions, and so on could be coming up in the future.
Honestly, you should be in the best position to answer this question yourself by referring to your organization's internal documentation of your org. If you don't have any such documentation, or don't trust it, I wouldn't turn on this setting without first deleting every single email address in the database first, and even that could be a daunting task.
It would be entirely impractical to try and research all the possible ways salesforce could have system processes and admin-defined processes that could trigger sending an email. For example, maybe there's a Campaign trigger that will email everyone when a certain field is set. Once you turn that setting on, anything's game.