The mail becomes spam and kills the company

Hi guys,

we had a serious Wp hacking problem and we sent a large amount of spam. Then ip was blacklisted.
Since then, yahoo, gmail, outlook and even the private courier companies are putting the email of our customers sent to them as spam.

we took these steps to restore the messaging service to normal.
1. cleaned the wp account and with the help of the customer, the site was secured.
2. From the IP list of blacklists except one. the request had asked for more details and remained in the proceedings, it seems.
3. We have replaced the IP address with a new IP address and configure all the records.
4. The new IP address is blank from any blacklist.

but email is still sent to spam on all networks. it's frustrating … our fully managed technical support indicates that they've done everything on the server side. asking me to contact Gmail, Yahoo and Outlook if the Builk email forms they have. I filled them but no luck.

what could the problem be guys?