We are currently using an old copy of core's emailing code for the following three things: target validation, sender validation, actual sending of the email. In particular:

• Target validation would be easy to migrate now: EmailUser::validateTarget works exactly the same as our CampaignsUserMailer::validateTarget. The problem here is that EmailUser::validateTarget is still @internal, and that's because it's still unclear where that method should belong. That's also the only thing left to do in T265541, but until that's done, we can't migrate it.
• As for sender validation: our copy of the code already calls User::canSendEmail, which uses EmailUser as of r921367. The replacement here is not 1:1 though: EmailUser ratelimits the sendemail right, but we need to ratelimit campaignevents-email-participants. Adding our ratelimit check should be easy enough. Removing core's check is harder, but maybe doable with the onPingLimiter hook. Note: just like core, we will need a method for the simple permission check and one for actually authorizing the action.
• Sending the email: the code in core is similar to what we need. There are a few differences though:
  • sendEmailUnsafe calls validateTarget. But in our case, target validation must be synchronous, whereas the email sending shouldn't (it's in a job). Duplicating the call shouldn't have any side effects. But also, given what I wrote above about ::validateTarget, it's still unclear how this code will evolve in core.
  • Core adds a footer about the email being sent via Special:EmailUser. We don't want this, and we may actually want a custom footer instead (T348043). Could be resolved by adding a method to EmailUser to replace/suppress/add the footer.
  • Core also adds a line to the footer linking to Special:Mute. I believe we want this.

All in all, it looks like the core side still needs a couple tweaks before we can use it.

Change #1023063 had a related patch set uploaded (by Daimona Eaytoy; author: Majavah):

[mediawiki/extensions/CampaignEvents@master] Use EmailUser class for permission checking

https://gerrit.wikimedia.org/r/1023063

Change #1023063 merged by jenkins-bot:

[mediawiki/extensions/CampaignEvents@master] Use EmailUser class for permission checking

https://gerrit.wikimedia.org/r/1023063