The pricing might be prohibitive for your usage. I don't know if any of them offer discounts/free usage for open source projects. (Not endorsing any of them; they've been sitting on my own list of things to investigate.)
Also, as an aside: SendGrid may not be suitable for transactional email you need delivered consistently quickly, unless you purchase their dedicated IP option. (And if you're bouncing a lot, a dedicated IP could actually be counterproductive for your delivery.) Recent incident [https://sendgrid.com/docs/Classroom/Deliver/shared_ip_thrott...] and discussion [https://news.ycombinator.com/item?id=12142728].