Outlook 2011 Keychain entry rewritten - prompting for password after 10.10.5 update - keychain issue?
The Exchange "autodiscover" settings changes the server URL to the WSDL resource.

So, mail.domain.com becomes: mail.domain.com/EWS/Exchange.asmx everytime you check for mail.

To fix this, you have to tell Outlook to stop asking for autodiscover settings 
using AppleScript editor:

tell application "Microsoft Outlook"
set background autodiscover of exchange account "NameOfAccount" to false
end tell


credit: http://www.officeformachelp.com/2011/01/exchange-autodiscover-workaround-for-advanced-settings/