I always thought that the IP address should be static. However, the docs say it is OK if it doesn't change more frequently than every 3 hours. That I have โ !
I am still having issues with my mails being delivered. That is mainly due to my ISP Pyur refusing to set up a PTR record for IP address & domain so that reverse IP lookup works. Without that, many email providers (rightfully) are skeptical about my server's authenticity and refuse my email.
Alternative: use a commercial email delivery provider