That’s quite an old topic, that feature was added in 2014.
That specific feature is about re-sending invites only, you can see it associated with invites you’ve sent on your profile: community.example.com/my/invited/pending
I don’t believe there’s a way to resend other emails from within the Discourse interface and it would needed to be done via command line on your server. I don’t know the exact steps for that, but depending on the importance of the missed emails, it may be more trouble than it’s worth.
I would love to know how to re-send failed e-mails, in fact, there should be a button one can click on the failed e-mail list to re-send once the e-mail settings have been fixed.
Or perhaps discourse should re-try a number of times/days on error.
It’s pretty important in my case, as I had hundreds of invites after I imported my old forum to discourse. Nobody got their invites!
I did however test e-mail functionality at at that time it worked fine. Somewhere between then and the time I did mass-invites mail stopped working. It was a server issue, but this could really happen anytime. I’m surprised there’s no way to re-send failed e-mails!