Why am i getting a pop-up 'sending failed' where ever i send an email?

[AC Question]

I am currently using Galaxy A7 2016 running on android version 7.0
I send a mail it shows sending and then fails always

 

[B. Diddy]

Welcome to Android Central! Which email app, and which email provider? Does this happen on both wi-fi and mobile data? If you're using the stock Email app, try removing the email account, then adding it back again.