Because you already have a full blown SMTP / MAIL server then actually rather than the hassle of 3rd party email hosting yourself is probably easier if you have a number of users.
But for that you really need a static IP so you can have a rDNS pointer with your domain name.
So there are two ways.
A. Hosting provider in essence what gmail is.
Email boxes on hosting provider and forward using an SMPT relay.
You have the email accounts hosted as there is a domain to send them to and in Zentyal setup up external accounts to download to the specific email user.
Sending uses the SMPT smart relay in the Mail settings.
This is a usual setup up if you don't have a domain name.
Also if you don't have a static IP with rDNS pointers to your domain name you will find emails returned to you or spammed.
B.
You have an domain name, static IP and your ISP will provide rDNS
Then setup up a virtual mail domain.
In your domain registry have a A record @ with your IP address of your static IP
Sometimes you don't even have to supply an MX record but this is stored in the DNS settings with the above.
I don't know what the relay policy is with the gmail smpt.