Office 365 allows a generous number of simultaneous connections per address, though not an unlimited one. When the same account is signed in across many mail apps and devices at once, they share that budget and a new connection can be refused.
BlueMail shows you the server's own wording when it happens:
Too many simultaneous connections for account [your address] (SMTP). Try again in 15 minutes
Waiting a few minutes usually clears it, because idle connections are released. To stop it recurring, sign in to myaccount.microsoft.com, go to Devices, and remove the ones you no longer use for this account.
