Too many simultaneous connections
If you have multiple users frequently accessing the same account or more than one IMAP client accessing Gmail at the same time, you may reach a connection threshold and receive the error message, 'Too many simultaneous connections.'
Gmail currently has a limit of 15 simultaneous IMAP connections per account.
Please note that your mail client may actually open multiple connections in the background. This means it's possible to reach a connection threshold simply by using only two mail clients to access the same account at the same time.
If you often use multiple mail clients or mobile devices with the same Gmail account, we recommend closing and/or signing out of any mail clients you're not actively using.