Note: To avoid temporarily locking yourself out of your account, make sure you don't exceed 2500 MB per day for IMAP downloads and 500 MB per day for IMAP uploads. If you're setting up a single IMAP account on multiple computers, try taking a break between each setup.

Turn IMAP access on and off

1. On your computer, open Gmail.
2. In the top right, click Settings .
3. Click Settings.
4. Click the Forwarding and POP/IMAP tab.
5. In the "IMAP access" section, select Enable IMAP.
6. Click Save Changes.  

More info

Turn on Less secure apps access

• Go to your Google Account.
• On the left navigation panel, click Security.
• On the Less secure app access panel, click Turn on access.

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

More info

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.

More info

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: Your name

Account Name, User name, or Email address: Your full email address

Password: Your Gmail 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/mail/answer/185833?hl=en