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!!!
Settings - Apps - Gmail - Storage and Clear Cache and Data. If you had custom notification sounds etc you'll need to set them up again - Also if you have more than one email account - Click on show all - You'd be surprised that sometimes you'll find an email just sitting there 🐧
Just so you know I did clear the cashe and data and it deleted all my email accounts. good job! That's when I had to call my provider and found out that I needed to click the SSL in security under email accounts