NoIP/DDNS/etc still means a dynamic IP address, with possibly broken reverse DNS, from a dynamic DNS pool.
To send email you need a static IP with correct reverse DNS, or other people's servers will reject your mail (best case) or silently mark it as spam. Welcome to the real world of email deliverability, the worst part of running your own mail server.
So use an SMTP relay service for outgoing mail. Most of them even have free tiers. I've been using one with a dynamic IP for years, albeit one where the IP doesn't change often.
On the receiving end I use a super inexpensive spam filtering service too, MX Guard dog. If my IP suddenly changes then it queues up mail until host resolution succeeds again.
To send email you need a static IP with correct reverse DNS, or other people's servers will reject your mail (best case) or silently mark it as spam. Welcome to the real world of email deliverability, the worst part of running your own mail server.