What do the error messages mean?
Cannot authenticate to IMAP server: AUTHENTICATE failed
Either the login name or the password you have entered is incorrect.
Cannot authenticate to IMAP server: [ALERT] Invalid credentials (Failure)
The error appears to be a way for Google Apps to protect against:
Robots or automated scripts
Stopping and halting abnormal bandwidth utilization (upload and download)
Abusing account activity (such as using Google Apps as a storage space drive)
Transferring another mail account to Google Apps
Incorrect password inputs
Cannot authenticate to IMAP server: [ALERT] Please log in via your web browser
The following errors may be due to the server configuration options that you are using. Contact your administrator for more details.
Cannot authenticate to IMAP server: [AUTHENTICATIONFAILED] Authentication failed
Cannot authenticate to IMAP server: [AUTHENTICATIONFAILED] Invalid credentials (Failure)
Cannot authenticate to IMAP server: [CLOSED] IMAP connection broken (authenticate)
[CLOSED] IMAP connection broken (server response)
What to do if emails stopped syncing?
Check the status of your email sync.
If it's Inactive, please reconnect. An inactive status is often caused by expired login credentials.
What to do if not all emails are syncing?
Check which emails are syncing and which are not.
If only incoming emails are syncing, there's a problem with the Outbox settings. In this case, the sync cannot find the mail folder that was mapped to Outbox. Please reconnect and sync the correct Outbox folder.
If only outgoing emails are syncing, check the Inbox connection.
If you're using Office 365: The default inbox is Inbox and default outbox is Sent Items. If you have customized your mailboxes, make sure to map Inbox to the folder that receives incoming emails you want to sync and map Outbox to the folder of outgoing emails you want to sync.