I know this no help, but you are set up exactly as I am. Occasionally, I'm will say authentication failed, but on resend will go. Hope you find the answer.
Thank you for your post.
CICT offered some great advice. The official document, FileMaker Pro Send Mail compatibility issues. The "Authentication failed" is the error FileMaker Pro is receiving from the Mail Server. You may want to contact the service provider if you still have issues after configuring as recommended.