"shared mailbox" is a specific Exchange feature for specific use cases -- e.g. all people in Sales having access to "sales@corp.com" mailbox.
So unless you need it, please don't fill it in.
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There are different ways to connect to Exchange, AquaMail uses something called "EWS" (Exchange Web Services), also used by Mail Droid, EmClient (Windows), Thunderbird (plugin) etc. -- but not ActiveSync which is used by Gmail app and "stock" Android Email.
It would be best to ask your corporate "computer people" -- whether EWS is in fact enabled (allowed) on your corporate server and if so, what settings (server settings, the login) you're supposed to use, as those can vary.