Try the following:

• Make sure you haven’t turned on email forwarding. If email forwarding is turned on, messages you receive might be going to a different account. To check, choose Preferences from the Action pop-up menu in the sidebar and in the General pane, make sure “Forward my email to” isn’t selected. If you deselect it, make sure to click Done.

• Make sure you’re using a browser recommended in the Apple Support article iCloud: System requirements.

• If you’re not seeing email from a particular sender, make sure you haven’t set up your mail rules to delete email from that sender. To check your email rules, do either of the following:

  • In iCloud Mail, choose Rules from the Action pop-up menu .

  • In Mail for OS X v10.7.5 or later, choose Preferences from the Mail menu, then click Rules.

• Make sure to check the Junk folder for email messages that were incorrectly marked as junk.