Hello kyam1,

I found a similar question thread from 3 years ago that had a solution that seemed to help the question owner. I hope it can help solve your issue.

Link to thread: Thunderbird sent emails go straight to spam boxes

The link did not work

It means you are sending mail which is getting caught by server spam filters.

Emails that contain too much remote content or too many links can get flagged as potential spam. OR if emails have inappropriate language or topics that are typically classified as spam.

If you use a signature then perhaps you need to use Write window to recreate another html file. You could remove signature and send to test if that html file is causing a problem. Do not use MSWord to create signature html.

Check to see if your email address or IP has got onto spamhaus filters. https://check.spamhaus.org/