Webmail Server required

[sagem2000]

Hello I was hoping that someone can point me in the right direction for a free self hosted mail server based on linux for Internal mailing as well as limited external mailing. My requirements are as below :

Locally hosted on Docker running on Asus NAS
Ability to send emails internally within an organization within the local subnet through LAN, without requiring internet.
Ability to SELECTIVELY allow only some accounts to send external emails (to gmail, hotmail, etc.) though an SMTP relay like Sendgrid, through internet.
Ability to block outgoing emails with attachments and to whitelist only certain email addresses and or domains.
Ability to SELECTIVELY allow only some accounts to ACCEPT external emails, from certain email id's or certain domains only.
Should also have a webmail like roundcube.

I find that hmailserver on windows satisfies all the above, but im looking for something that will run on docker on Asus Nas.

The more GUI available for configuration, the better

[Nazar78]

There's a Mail Server in the app central. It's actually postfix and quite powerful only that many settings are not in the GUI and you should know how to use it in the first place. I'm running postfix with DKIM in my router instead. For the webmail client you can setup any client you prefer using docker then set it up with your local postfix.