Since upgrading to OSX El Capitan, I noticed one of my POP accounts keeps going offline. That account is hosted on Godaddy and after some web trawling it seems that Godaddy hosted POP email accounts won’t play nicely with Apple Mail under El Capitan. This may be the case with other hosting providers as well.
Thankfully, the solution is simple. Go to Preferences in Apple Mail and under the ‘Advanced’ tab there’s an option labeled ‘Automatically detect and maintain account settings’. If you
uncheck that box, your account will start working again.
Here’s a screen shot example:
I hope that’s of some help!
Had same problem, all my accounts go offline after a few minutes, to get them back online I have to turn the wifi on and off then they work. But them they go offline in 15 minutes.
So i have done what you recommended, that is uncheck automatically detect account setting, then restart mail. Yes it worked but unfortunately it still goes offline in 15 minutes. Its a little better but does not fix the problem
mac mail is now CRAP.