HTML emails fail DKIM validation: "body has been altered"

[xorinzor]

When I send an email using my local mailbird client the DKIM validation passes, yet when I send an email using the wordpress "WP Mail SMTP" plugin it fails with the error that the "body has been altered".

Reading up online this could potentially have to do with the body hash calculation where line endings might not be parsed correctly, but I am unsure how to debug if this is the actual cause.

Both my mailbird client and the SMTP plugin use the same credentials and same port (465). I also tried using port 25, but it provided the same results.

Is this a bug in the mail server, or do I possibly have something misconfigured?

success - dkim validation.txt
success - original_message.txt
failed - dkim validation.txt
failed - original_message.txt

[xorinzor]

Just discovered it might possibly have to do with a missing content-transfer-encoding header.

Will need to dig a little deeper on this. Reference: https://stackoverflow.com/a/28531705/1024322

EDIT: Fixed it by using "Post SMTP" instead of the "WP mail SMTP" plugin. Now the header is being added correctly.