Entourage implies you are using a very old version of Office probably Office 2008. Microsoft no longer support Office 2008 or earlier versions, this means they will not fix any issues with it. The current version of Office for Mac is now Office 2016. (The clue to the age of Office 2008 is in its name 🙂)
Currently there are known problems between Office 2016 and El Capitan (aka. 10.11) which Microsoft are aware of and working to fix. Outlook for Mac in Office 2011 was the replacement for Entourage and while Outlook 2011 initially also had problems with El Capitan these are supposedly now fixed in the Office 2011 14.5.6 update which Microsoft recently issued.
So, if your using Office 2008 or earlier you will need to buy an upgrade, if your using Office 2011 you can download and install a free update, and if your using Office 2016 you need to wait for a fix.