Most hosting services offer email accounts to their clients. If a member has an account with Host X , but changes to Host Y, then the account with Host X will become void, The member may be loyal to your site, but simply forgot to update their new email address. It happens all the time.
if you send out a bulk mailing and get a bounce from Host X, does it mean your loyal member has ditched you?
No!
If it concerns you that much, you could message the member and tell them that a recent email bounced. You could politely say that they have x days to update their email or you will suspend their account.
Personally I wouldn't bother. What's more important is when the member last visited the site. Was it last week or a year ago?
I have a small free mod that allows you to see when a member last visited their profile. I would consider anyone who hasn't visited for a year as someone who's no longer interested in the site. I'd be deleting or suspending their profile under those circumstances.
https://www.boonex.com/forums/topic/Profile-Visits-View-Counter.htm