Which kind of account you are using?
If you are using a business account （ for example firstname.lastname@example.org) you must set POP account info properly, filling all the blank fields (name, email address, description, password, Host Name, user's name, SMTP). Just one field blanked causes error in sending/receiving.
If you are using a default account (iCloud, Yahoo!, Hotmail...), fill all the blanck fields (Name, Email account, Password, Description） and sign in.
If not works, delete the account and sign in again
Hope it will be helpful