Apple has a unique way of identifying the default email and in certain devices (Example IOS 11) the setting of the default email client is not as easy as one setting and requires you to set the default for multiple email systems.
\uD83D\uDCD8 Instructions
Go to your phone settings and click on Mail.
Go to Accounts:
Click on Add Account:
Select mail type what ever your work email is using I select exchange.
Provide you credentials and Add your account:
I have added my work email showing in the list as exchange:
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