Gmail allows 15 simultaneous IMAP connections per account, which is generous but finite. If the same address is set up across several mail apps, computers and phones at once, they share that budget, and the newest one to connect is the one turned away. Freeing a few connections puts everything back to normal.
BlueMail passes the server's own wording straight through when it happens, so you know it came from Gmail rather than from the app:
Too many simultaneous connections for account [your address] (SMTP). Try again in 15 minutes
Waiting a few minutes clears it on its own, since idle connections are released. For a lasting fix, review what still has access on Google's third-party connections page and remove anything you no longer use. Old mail apps on devices you have replaced are the usual culprits.
