The Gmail IMAP & POP-related issues refer to problems encountered while configuring or using IMAP & POP settings in Gmail. These issues may include difficulty in syncing or accessing Gmail on external email clients or devices, authentication errors, and incorrect configuration settings. These issues can be resolved by ensuring the correct settings are in place and troubleshooting the device's connection to the email server.

Causes

• Incorrect IMAP or POP settings configuration
• Issues with the email client application
• The outdated email client software
• Gmail server issues or maintenance
• Incompatible third-party apps or add-ons
• Account sync issues with multiple devices
• Internet connectivity problems.

Solution 1: Ensure that you have enabled the IMAP/POP settings in Gmail

1. In the beginning, log in to your Gmail account on a web browser.
2. Next, tap on the gear icon located in the top right corner.
3. After that, select Settings from the drop-down menu.
4. Then, open the "Forwarding and POP/IMAP tab.
5. Under the IMAP Access section, select Enable IMAP.
6. Under the POP Download section, select Enable POP for all mail or Enable POP for mail that arrives from now on, depending on your preference.
7. Press Save Changes at the bottom of the page to enable the settings.
8. Once you have enabled these settings, you should be able to access your Gmail account using IMAP or POP on other email clients.

Solution 2: Check login details and server settings

1. First, double-check your email and password to make sure they are entered correctly.
2. Ensure that you have entered the correct server settings for your email client, such as the incoming and outgoing server addresses, port numbers, and encryption settings.
3. If you're not sure about the correct server settings, you can check Gmail's help documentation or contact your email client's support team.
4. If you've recently changed your password, make sure that you've updated it in your email client's settings as well.
5. Finally, try logging in again with the correct credentials and server settings to see if the issue has been fixed.

Solution 3: Disable any antivirus or firewall that may be interfering with the email client

1. To get started, open your antivirus or firewall software program. 
2. Find the settings menu and select the option to disable or turn off the program.
3. After that, save the changes and close the settings menu.
4. Try accessing your email client again to see if the issue is solved.
5. If the issue persists, contact the antivirus or firewall provider for further help.

Solution 4: Try using a different email client or app that supports IMAP/POP protocols

1. Firstly, choose an email client or app that supports IMAP/POP protocols.
2. After that, install and launch the email client or app on your computer.
3. Then, add your Gmail account by entering your login details and server settings.
4. Test if you can access your Gmail messages on the new email client/app.
5. If successful, you can continue using the new email client/app or troubleshoot the issue with your previous email client/app.