Maybe DDNS will help you.
It tracks your floating IP, and maps it to a constant DNS name, otherwise you may be able ro pay your ISP $5// month to assign you a static address.
From your question, I’m assuming you’re running an onsite mail server, not a publicly visible server hosted elsewhere.
Typically one doesn’t send email to am IP address, you send it to a mail,server that is identified by the MX records in the publicly visible DNS records.