Can't get IMAP accounts to work in both macOS and iOS
I have five IMAP accounts with FatCow, which hosts my websites. I have been able to get one to receive mail in iOS. Same credentials don't receive mail in macOS. Two others have been verified, or don't give an error in macOS, but also don't receive messages. Two cannot be verified in macOS, though FatCow conforms the credentials are correct.
Does anyone know what's going on, or what can be done?
Thanks.
UPDATE: All five accounts are verified and functioning in iOS.
MacBook Pro, macOS High Sierra (10.13.5), Late 2016 15" 2.7 GHz i7 16 GB RAM