See File/Preferences, then Mail, Sending and Receiving, Check for new mail
Don't set it to 1 or 2 minutes, it's useless. For messages that urgent there is Skype. The default of 15 minutes is usually okay. By the way, I noticed that R8.5 signals much faster than the 15 minutes. If the mail doesn't appear immediately in the Inbox, the user might have to type the F9-key to refresh the Inbox.