Problems sending mail

I have an iPhone 6 running latest software iOS 8.3 for some reason I am able to get all my mail on my inbox daily but when I try to reply, forward or compose a new email a box pops up with a message saying: Cannot Send Mail The user name or password for (my email address) is incorrect. I know the right password has been entered because when I go to the settings I enter it and it takes it. I tried entering a bogus number and it automatically tells me that I entered the wrong number.

Also when I go on my PC and try to go to the page of my mail which in this case is AT&T I am able to log in with the password I am entering on my iPhone. So I ask this community what is wrong? I'm I doing something wrong, is it a glitch? I already tried deleting my mail account and added it again and still having the same issue.

Help please, thanks.