MailChannels have recently announced that they will be discontinuing their free API service with Cloudflare Workers as of June 30th. You can find details here:
They also stated in the Cloudflare Discord server that:
Starting next week, you may notice a small fraction (1%) of your connections are rejected with an error message containing a link to the same announcement. Any messages that are accepted by the service will be delivered as usual and we will continue to provide support as usual until the final day of service.
So I would recommend folks migrate now!
Alternatives
I can confidently recommend both Postmark and SendGrid, and have heard good things about Resend.
The Cloudflare developer docs has great tutorials for sending with both Postmark and Resend:
Wow this was quite a stab in the back. I literally learned about it today when I looked at the logs and saw all those errors. At least sending a warning email in advance would be nice
This is very frustrating. I’ve spent lot of time debugging why transactional emailing wasn’t working anymore in the app we’re building. The fact that workers supported sending emails out of the box was a primary reason for deciding to use cloudflare workers. That just sucks.