Mailgun reduced their free tier drastically recently, I think to 625/month. If that fits your needs it's a decent choice.
HN discussion from when they downgraded the free tier, includes some discussion of alternatives: https://news.ycombinator.com/item?id=22192543
Link from the email they sent: https://help.mailgun.com/hc/en-us/articles/360048661093-How-...