Outlook and Mac Mail do not sync with Exchange 2010

Dear Experts,
I have one Mac user among 70 PC users, and am having an incredibly difficult time making his mail to work.
His OS is Yosemite 10.10.1, and Outlook for Mac is version 2011, and neither of them will sync properly, after reinstalling Office, cleaning out Keychain, recreating accounts over and over, etc.
Outlook will sync initially when I create the account, then will keep saying "Mail could not be received at this time. ...unknown user name or bad password...." which is not true because it syncs initially, and I have checked, changed password to see if that makes any difference, but no.
Apple Mail will work, but the Inbox will get synced whenever, and incoming mails are delayed up to 3 hours at a time.  Nothing is on time.

I have been trying to deal with these issues, the user has to rely on his iPhone in order to get regular mail.
Please advise.
yballanAsked:
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dhsindyRetired considering supplemental income.Commented:
See paragraph 4 about the devices "losing" server settings: http://www.techrepublic.com/article/pro-tip-troubleshooting-10-known-os-x-yosemite-issues/

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yballanAuthor Commented:
Thank you, it is at least good to know that this symptom has been recognized!!
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