Go here:
and here
and maybe here
https://whatismyipaddress.com/blacklist-check
find out what lists you’re on, and try to get off them. Your hosting company / datacenter might be able to help.
Then prohibit your clients from doing mass-mailings from your server. Let them use Constant Contact, Mailchimp, or whatever. If they whine, non-renew them. They’re not worth the aggravation of having to constantly get off blocklists.
Richard