Using the Gmail app on the S7 edge (and I think the same thing happens if I use the Samsung mail app).
There appears to be an email in the Outbox folder that is failing to send. I see that message in the notifications.
But if I look at the Outbox it says there is nothing there. Says the same thing if I go to Gmail on a browser on my desktop PC
It keeps trying to send this email and about every minute or less the phone beeps - it's the default notification.
I turned off everything else - there's no other reason for this beeping.
I rebooted the phone, no help.
I deleted the gmail account and re-added it, no help.
I went to flight mode so it couldnt try to send the message and I still couldnt find the message that's having trouble.
I'd be glad to just delete the thing and carry on - just want the incessant beeping to stop!!!