Mail Issues with High Sierra

Upgraded my 2011 iMac to High Sierra, now having Mail problems. Was running OS X Lion 10.7.5, in order to upgrade to High Sierra, first needed to upgraded to OS X El Capitan 10.11.6 then to macOS High Sierra 10.13.1. Now that I'm at High Sierra, I'm having Mail issues. I have 3 address. All worked fine prior to the upgrade. After the upgrade, none worked. Kept getting a message stating accounts are off line. I would try going in and to put the accounts back on line but it would not work. So I tried to just re-add the addresses as if they were new and that worked for two of them. The third did not. So after trying several times I deleted the address that was not working and tried to re-add it. When I try to re-add the address a message comes up saying "This account already exists". Is there anyway to get the account back up on Mail? Thanks.