How to work with domain mail on two mail servers at the same time?

Question

Domain Name System

There is G Suite, which processes mail coming to the domain, and there is a mail server based on Mailcow.
You need to configure this mail server so that through GSuite you can still work on several mail accounts, and all letters to the rest fall on your own mail server.
As I understand it, a simple change in MX is not enough and you need to set up forwarding?
How to do it right?

Answer 1

Initial state.
1. Mail receives and decides where to send - Google. SPF MX DKIM dns records are initially configured on it.
2. The mail server is deployed on Digital Ocean and is pristine.
Applications => G Suite => Gmail Settings => Advanced Settings
1. Added in the "Hosts" tab - a new Mailcow host with the corresponding mail server address.
2. Added in the "Default routing" tab - change the route for all recipients . I chose Mailcow as the route from point 1. This option applies to all unrecognized addresses

why is that

По условию задачи, в google нужно было оставить как раз те 3.5 аккаунта, а остальные убрать на свой сервер. В данном случае нераспознанные аккаунты это все адреса почт, которых нет в google

3. Finally, in the "General Settings" tab, changed the SMTP Relay Service . By adding there only senders from my domain from the address of the mail server .
setup 1. Changed the postfix setting in the main.cf file:
relayhost = smtp-relay.gmail.com:587
2. Created an account [email protected] on mailcow.
Everything started up.
Google receives letters and when it sees that there is no corresponding account (mail address), it forwards the mail to the mail server.
The mail server itself does not send mail, but does it through Google's smtp.

Answer 2

The configuration is valid. A common pattern in the Exchange corporate world is to use an Internal Relay (Accepted domain type).
Those. the idea is this, do MXs on your mailcow, do not make your domain Authoritative, but change it to Internal Relay, the idea is as follows, if the mailbox is not found in the mailcow, it sends mail to G Suite.
Needless to say, how to set up a similar mode and whether mailcow will work - you need to do it yourself.