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There are a handful of (commercial) email address verification services, which could automate some of the logic for you:

* https://kickbox.io * https://www.emailhippo.com * http://www.briteverify.com

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].

Oh, also, another resource if you want to roll your own email verification: https://www.scottbrady91.com/Email-Verification/Python-Email...

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