If your G Suite users want to use EmailTree to access their G Suite mail, you need to enable Internet Message Access Protocol (IMAP) access in the G Suite admin console. You can enable access for everyone in your account or only for users in specific organizational units.

Google Admin console - Settings

Turn IMAP access on and off

You can enable access for everyone in your account or only for users in specific organizational units.

1. In your Google Admin console (at admin.google.com)...
   • Sign in using your administrator account (does not end in @gmail.com).
2. Go to Apps > G Suite > Gmail > End User Access.
3. In the Organizations section, select the organizational unit you want to configure settings for.
4. Next to POP and IMAP Access, check or uncheck the Disable POP and IMAP access for all users box. If you check the box, POP and IMAP are fully disabled for all users in your domain.
5. Users can't access POP and IMAP settings in the Gmail interface.
6. Users can't access their mail via POP or IMAP even if they originally had it on.

If you uncheck the box, POP and IMAP using modern security standards is enabled for your users, but IMAP for use with less secure apps remains disabled.

• POP access is automatically turned on for your users.
• To use IMAP with less secure apps, users need to enable IMAP in their Gmail settings under Forwarding and POP/IMAP, and then follow the instructions to Let less secure apps use your account.

7. Click Save changes.

Note: It can take up to 24 hours for IMAP and POP changes to take effect. As long as the box is unchecked, users can configure POP and IMAP access for any supported mail client.

More info

Note: If username and password aren't recognized, it's needed to turn on access for less secure apps before setting up POP/IMAP connections.

Turn on Less secure apps access

1. In your Google Admin console (at admin.google.com)...
   • Sign in using your administrator account (does not end in @gmail.com).
2. Go to Security > Basic settings
3. Under Less secure apps, select select Go to settings for less secure apps.
4. On the left, select an organizational unit where you want to manage access to less secure apps.
5. If you don’t select an organizational unit, your setting applies to your entire top-level organization.
6. If you want an organizational unit to use the same setting as its parent organization, click Use Inherited on the top right.
7. Select the option Allow users to manage their access to less secure apps
8. On the bottom right, click Save.

More info

G Suite Gmail - Settings

Enable IMAP for your account

1. On your computer, sign in to Gmail with your G Suite account.
2. In the top right, click Settings  > Settings.
3. At the top, click Forwarding and POP/IMAP.

Don't see this tab? Make sure IMAP is enabled for your domain.​

4. In the IMAP Access section, select Enable IMAP.
5. At the bottom, click Save Changes.

Turn on 2-Step Verification

Note: App Passwords can only be used with accounts that have 2-Step Verification turned on.

1. Go to your Google Account.
2. On the left navigation panel, click Security.
3. On the Signing in to Google panel, click 2-Step Verification.
4. Click Get started.
5. Follow the steps on the screen.

Choose a second verification step

When you set up 2-Step Verification, you can choose your second verification step:

• Phone prompt
• Text message (SMS) or call
• Security key

Create & use App Passwords

1. Go to your Google Account.
2. On the left navigation panel, choose Security.
3. On the "Signing in to Google" panel, choose App Passwords. If you don’t see this option:
   • 2-Step Verification is not set up for your account.
   • 2-Step Verification is set up for security keys only.
   • Your account is through work, school, or other organization.
   • You’ve turned on Advanced Protection for your account.
4. At the bottom, choose Select app and choose the app you’re using.
5. Choose Select device and choose the device you’re using.
6. Choose Generate.
7. Follow the instructions to enter the App Password. The App Password is the 16-character code in the yellow bar on your device.
8. Choose Done.

Most of the time, you’ll only have to enter an App Password once per app or device, so don’t worry about memorizing it.

Note: To help protect your account, your app passwords are revoked when you change your Google Account password. You can always create a new app password to continue using an app with your Google Account.

Set up Emailtree to access Gmail

You might need to provide the following details:

Your Account information

Full name or display name: This is how your name appears when you send email
Your G Suite account details: Your username and password

Incoming mail settings

Server: imap.gmail.com
Require SSL: Yes
Port: 993

Outgoing mail

Server: smtp.gmail.com
Require SSL: Yes
Require TLS: Yes
Require authentication: Yes
Port for SSL: 465
Port for TLS/STARTTLS: 587

Source: https://support.google.com/a/answer/105694?authuser=1