I've been experiencing the same issue since this weekend.
Here are some posts from people having similar issues I found:
* Apple Support Community
- Gmail emails are delayed or not sending to iCloud
https://discussions.apple.com/thread/6873382
- HT201771 Over the weekend my @me.com email address was failing to communicate with @gmail.com email addresses. What needs to be done to fix this?
https://discussions.apple.com/thread/6877788
- Why are emails to my me.com address bouncing back to the senders?
https://discussions.apple.com/thread/6872650
* MacRumors Forum
- iCloud email - Issues Receiving:
http://forums.macrumors.com/showthread.php?t=1856290
* Gmail Help Forum
- My Emails Aren't Being Received By iCloud Users:
https://productforums.google.com/d/topic/gmail/oPb9pnW5vN8/discussion
- from a gmail address to an iCloud address is delayed:
https://productforums.google.com/d/topic/gmail/BJNCkQz2Geg/discussion
- gmail messages slow to be received by iCloud:
https://productforums.google.com/d/topic/gmail/tUuioFAe64U/discussion
- Gmail forwarding to my iCloud account:
https://productforums.google.com/d/topic/gmail/MP_doUYgVEM/discussion
- GMAIL ERROR 451 4.5.3:
https://productforums.google.com/d/topic/gmail/ZkhG4HxVNyM/discussion
- Problem with gmail sent to a group either having a delayed receipt or failed receipt to any mac/me/icloud addresses:
https://productforums.google.com/d/topic/gmail/PSHqUV0YpZQ/discussion
- Delayed Emails to @Me domain HELP:
https://productforums.google.com/d/topic/gmail/XYjPX_iunvQ/discussion