I am referring to the client having a dynamic IP connection with their ISP - The client's IP address may change for their email server. So DDNS service provide

I am referring to the client having a dynamic IP connection with their ISP - The client's IP address may change for their email server. So DDNS service providers (i.e. ChangeIP, etc.) handle DNS for those - An application locally installed for the client (Synology offers one for their NAS devices, for example) polls the public IP every few minutes (or some frequency), then reaches out to the DDNS provider with any updates, that provider in turn updates DNS so the rest of the world "sees" the email server at the new address.