Regular Gmail synchronization issues:
Updated: As of lately Google has started disabling IMAP by default so I will show you how to turn it back on.
First click on the settings>See all settings button.
From this page please click on Forwarding and Pop/IMAP and then make sure IMAP is enabled.
Let's make sure that your 2-Step verification login is off and Allow less secure apps is on.
To do this login to your Google account and then click on your profile in the top right then click on Google account.
From this screen click on security and then you will see both options to make sure 2-step verification is off.
And then at the bottom of the page turn on less secure apps.
Here is Video:
This should fix any issues that would be stopping your google account from syncing.
However, If you are still experiencing issues, especially the error message "web login required" visit this link to allow access to your Google account.
Here are some Google Articles on this issues as well:
Keeping 2-step Authentication on with App Passwords:https://support.google.com/accounts/answer/185833?hl=en
Instructions to enabling IMAP: https://support.google.com/a/answer/105694?hl=en
Less Secure Apps: https://support.google.com/a/answer/6260879
Another Article for IMAP: https://support.google.com/mail/troubleshooter/1668960?rd=1
G Suites Users:
If you are a G Suites user it is the same fix their menus are just set up differently.
First, you will want to navigate to admin.google.com and sign in. From here you will see this page please select security.
Now go ahead and select the option basic settings.
Then click on go to settings for less secure apps.
Now from this last page you will either select enforce less secure apps for all or allow the user to enable them. If you select allow the user to enable them please then follow the Gmail instructions above to finish the syncing.
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