What causes high spam score?

No. Spam score is made up metric created by Moz and has no affiliation with Google.

You can see a more in-depth answer here: https://seochat.io/question/what-is-mozs-spam-score/111

1. Create a text version.
2. Avoid usage of spam sensitive words and phrases.
3. Use normal text inside hyperlinks, instead of the URL.
4. Use a proper from address.
5. Use a proper subject line.
6. Avoid HTML errors, unsupported techniques, obscure content, obfuscation.

• A number of Domains Linked-To Spam sites are more likely to have abnormally high or low unique domains to which they link. 
• The ratio of External Links to Content Spam sites are more likely to have abnormal ratios of links to content.