Mail randomly disconnecting from servers

I have two mail accounts, one Exchange and one Yahoo. Both seem to disconnect from their servers, never at the same time and almost randomly. I get these messages asking if I want to retry sending through the appropriate server; when I click yes the mail almost always goes through. Really don't get it. I don't have these problems with either my iPad or my iPhone with the same accounts, and those accounts both promptly push email whereas on the iMac it's delayed consistently. I also have similar problems with the calendar app; starting an appointment I frequently get a message saying I'm not connected with the Exchange server so the appointment doesn't sync all the time. Seeing as I just recently switched from Windows, my major disappointment with the Mac are the mail and calendar apps. Is there something I'm missing, or is this the usual amount of dysfunction?