Worker -> MailChannels -> email routing = SPF permerror

This is sadly not a direct solution to your issue, but 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, that have still not truly acknowledged the problems, and brushed it aside or “patched” it with paywalls.