Below are my recommendations for free email marketing services that are also easy to scale up, if your user base really picks up. I'd start up with either SendinBlue, Sender or MailerLite - no one says you cannot use 3 of them at once, if you split your lists correctly you could manage 3 different ones and benefit from all their limits. The comparison below only shows the limits I'm talking about, but in terms of features most of the ones below have pretty much everything you'd need to run successful email marketing campaigns. So focus on limits, as all the features you'd need are also in the free tiers.