It'll detect if IMAP IDLE is supported on the provider and use it wherever it can, so IIRC: Gmail / iCloud: Yes FastMail: Yes Yahoo: No AoL / Exchange / Hotmai

It'll detect if IMAP IDLE is supported on the provider and use it wherever it can, so IIRC: Gmail / iCloud: Yes FastMail: Yes Yahoo: No AoL / Exchange / Hotmail / Live: Can't remember