Spam Assassin reports PP_MIME_FAKE_ASCII_TEXT MIME text/plain claims to be ASCII but isn't

Question

When emailing via Sendgrid SMTP from Gmail using https://www.mail-tester.com/ it reports SpamAssassin says "PP_MIME_FAKE_ASCII_TEXT MIME text/plain claims to be ASCII but isn't".

Sending the same content from the Gmail address via Gmail's SMTP servers to mail tester does not give this error.

What is going on? How do I fix this error?

Update 2021-03-07 Sendgrid say they require the "SMTP payload" sent from Gmail to their SMTP server. Any suggestions how I can get this?

Thanks

Answer 1

An email entity that claims to be 7bit text contains 8bit characters.

Maybe Google is fixing it for you and sendgrid isn’t.

Check the source of the email you are sending and compare it with what you receive from Google.