iCloud mail hanging up
iCloud mail hangs up when I try to view a message. the box itself is loading; I just freezes when trying to prepare the message view, and then I can't do anything in the mailbox. Very frustrating.
If this is happening in the Mail application, try rebuilding the mailbox.
Rebuild mailbox
If that doesn't work, try re-indexing the mailboxes. This can take awhile if you have a lot of mail.
Reindex messages For El Capitan, try looking in V3.
Reindex messages (2) See post by Linc Davis