My email forwarding[1] is the one I most proud of. As every engineer, I had a bunch of side projects with many domains, anytime it comes up I have to do the dance of setting up email.
I tried use some existing email forwarding service but always something come up. I decided to roll my own and eventually start a SaaS.
The second ons is my newsletter[2], it runs on my custom static site generator. It use mailchimp to store user data since I don't want to deal with GDPR etc. But all the actual email handling is using my own custom code.
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[1]: https://mailwip.com