Problem to receive mail from my customer.

Dear all,

I would like to inform you that i am facing the following problem. One of my customer had a problem with one of his public mail server that was blacklisted because of a spam issue. Customer inform us that the issue has been resolved but some servers still keeping this public ip on their blacklist database and refused mail coming from them. My organization has also problem to receive mail from them. Could you please inform me what i should do so as to check if his IP is on blacklisted servers and what steps i should execute so as to resolve the issue.

Thanks in advance.

BR,