Even though I'm no longer having the problem, I did just stumble upon a possible answer (I'm pasting in below); message is dated 12/11/14. So maybe it is iCloud and maybe if you report it to apple, as I did, your problem will resolve as mine did. Pam
Link is here: https://productforums.google.com/forum/#!topic/gmail/rYk8qoO2TyY
"Hi Jeff,
Andy from Google here. We looked into this and found the following error message occurring two times for that message that eventually delivered:
"TEMP_FAILURE: #1: Google tried to deliver your message, but it was rejected by the server for the recipient domain icloud.com by mx1.mail.icloud.com. The error that the other server returned was: Temporary lookup failure: jjm**...@icloud.com"
So it appears it's an issue with one of icloud's mail servers, a "temporary lookup failure", to that particular address when we are trying to deliver the message. Went ahead and double-checked on our side as well, and we haven't been experiencing any message delivery issues that would lead to this delay.
Hope this is helpful, and thanks Brett for raising! Andy"