Is there a performant/scalable way to email all existing Meteor users or should I be using another service to do something along these lines?
I would definitely outsource the sending, using a service like Mailgun for example.
It depends on how you want to send out those emails. You could generate them just on the server. But in case it’s a big job you could also build some separate app which handles generating those emails.
Because you only have to send them to Mailgun you can do quite a lot of them just on the server without performance issues.
Do you need it for mailings or do you have a different use-case?
Mainly just for releasing product info announcements, tips for using the product, and emergency updates. Things of that nature.
Then it might be even better to sync the list of e-mails with a provider like MailChimp so you get all the tools to manage e-mail.