Sendgrid is the cause of nightmares, it’s not uncommon that customers use Sendgrid, and it’s quite understandable with a “free tier” that allows you to send e-mails from your application for free.
But what Sendgrid lacks is proper Spam-mitigation, since Sendgrids entry level “free tier” and their first level of payed service uses a shared service platform, this means that it only takes one of Sendgrids customers to misbehave (and they do!) for all other customers using that same tier to be affected, causing e-mails to be blocked by the customers spam-mitigation filters or even worse being blacklisted for extended periods.
The solution in the Sendgrid world, is to upgrade to their highest tier, currently at ~90 USD per month. But why did we get stuck here? Well it was because of the free tier in the beginning, because ~90 USD per month, is quite pricy when alternative services pricing starts around 8 USD per month (depending on volume).