An IP address can sometimes get associated with spam because a previous user of that address has been prolific spammer, resulting in spam filters screening out all email from that address indefinitely.
So try changing your IP address. In most cases that involves nothing more complicated than removing the power to your router, waiting a minute or two and then powering it back up again.
To see what your current IP address is, click here
https://whatismyipaddress.com/
Then repeat after power-cycling your router, to confirm that it's changed.