To everyone experiencing this problem on both iPhone and Mac, what is your Incoming Mail Server setting under Mail App > Preferences > Accounts > Account Information on your Mac?
iPhone does not have the ability to check your Incoming Mail Server, I believe.
I ask because I own two iCloud accounts, one on p99-imap.mail.me.com, and one on p01-imap.mail.me.com.
p99 works completely fine, I am able to send and receive email on iPhone, Mac, as well as www.iCloud.com.
p01 on the other hand, does not. It is able to send outgoing emails on iPhone, Mac, and www.iCloud.com, but is unable to receive incoming emails on either iPhone or Mac. However, www.iCloud.com is able to both send and receive.
This leads me to believe that there is a problem with the Incoming Mail Server p01-imap.mail.me.com, which appears to only affect iPhone/Mac and NOT www.iCloud.com. The Outgoing Mail Server does not appear to be affected at all.
Could everyone please confirm if you are experiencing the same issue so that we may narrow down the source of the problem for Apple?