Mailchannels stopped working for free, always get 500 response code

You’re likely hitting the Domain Lockdown feature, which was added after MailChannels was discovered to be a security nightmare and something essentially no one should use.

I would recommend avoiding MailChannels and using some other service like Amazon SES, Sendgrid, Postmark, etc.

There are a lot of security implications and their responses to the situation have been very undesirable. Further reading:

To this day, they have still not truly acknowledged the problems, and brushed it aside or “patched” it with paywalls.