@Marc Stridgenalways write that you have thousands of satisfied customers to justify any complaints ...
Search sendgrid in this forum and you will see hundreds of messages from people having problems ...
Just for the forum we give you 250 dollars and every 6 months another 40 (without considering that you have bought blogs, ecommerces, galleries, etc.), it would be the minimum to develop api for free services like Amazon AWS and not to force users to subscribe to services for a fee to send emails.
There are many other great services such as mailcimp, sendingblue etc. why don't you develop api for them since it is now certain that sendgrid has problems? Sendgrid even, with the first paid plan, "No Access to Email Validation" ...
If you have thousands of customers it means that you earn millions of euros a year, developing options so as not to make your customers spend more money is the minimum ...
And it's minimal to even develop a simple prefix system like all free forums have ...
What I noticed instead, is that knowing that you have little competition (only xenforo) you are simply not interested in improving your forum ...