I assume you're talking about the "bad email" status that some accounts have. (OK, it's not just "some", there are 9,650 accounts with that status
It generally means the email on their account doesn't work anymore. In other words, the site got a "bounce" back when it emailed them. In that case, the account is made inactive. They would have to sign on to it and fix the email in order to activate it (something like that). It's fairly common for emails to stop working, so I have that status to try to deal with it. Having a valid/working email on an account is important.
I actually disabled the automatic thing where it changes the status when a bounce happens a few years ago. So, these accounts generally haven't been used for years.
No, it's not for people in the process of switching emails. That is the "mail check" status. There are 70 accounts with that status at the moment.