Problems replying to email from workers

I'm following https://developers.cloudflare.com/email-routing/email-workers/reply-email-workers/ and that code works verbatim and sends a reply. However, testing this from my own email I keep seeing sporadic error that says "original email is not repliable".

What does this mean? How come some of the test emails I'm sending from my gmail are repiable and some others are not?
Cloudflare Docs
You can reply to incoming emails with another new message and implement smart auto-responders programmatically, adding any content and context in the
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