The most likely reasons your email is going to spam are as follows.

1. You didn't get permission to email
2. Your IP Address was used for spam
3. You have low engagement rates
4. Your subscribers don't remember you
5. You have low Mailbox usage
6. Your subject line is misleading
7. Your 'from' information is inaccurate
8. You didn't include a physical address
9. You didn't include an 'unsubscribe' link
10. You used span trigger words
11. Your HTML emails don't follow best practices

See more detail here: https://optinmonster.com/11-reasons-why-your-emails-go-in-the-spam-box-and-how-to-make-sure-they-dont/

How to remove from spam lists:

https://www.techwalla.com/articles/how-to-remove-your-email-address-from-the-spam-list
https://www.spamhaus.org/consumer/removelists/
https://support.google.com/mail/thread/5543913?hl=en