Fix Mail Problems
Even though Mail in El Capitan is more reliable than its predecessors, problems will still occur. I’ve compiled solutions to some of the most common (and serious) problems in the pages ahead, including difficulties receiving and sending mail, problems with damaged mailboxes, and misbehaving Spotlight searches.
Although some of this information may provide useful background for resolving iOS Mail issues too, the specifics are different; see Troubleshoot iOS Mail Problems for help with Mail in iOS 9.
Fix Incoming Mail Problems
If Mail cannot download incoming messages, the cause may be anything from an incorrect setting in Mail, to a server problem, to a loss of Internet connectivity. Work through each of the following sections ...