Is the mail account actually configured as Exchange / EWS on the CEO's phone? Is the Exchange tab there at all in Aqua's contact picker?
To check:
- Long press the account
- Account setup
- Manual
You'll be looking at incoming (and for Exchange, the only) mail server settings. The server type / mail protocol is shown near the top: IMAP / POP3 / EWS.
To reconfigure: delete the account from Aqua, add back in, making sure to select Exchange (not Internet) when prompted.
One more thing: check which account is selected in the compose window when bringing up the contact picker. This is a drop-down in the action bar (title bar) of the compose window.