As far as I know, there is no way to make any email app a default on your Android phone. I use both Gmail and Outlook as my email apps, and neither one are a "default."
There is no default email app. You can use whichever or however many mail apps you want. If you are trying to make it the default as to which app will open when you click a link on the web to email, I believe the app chooser will come up to ask you which app/email account to use. If you want to share via email, the share tap should give you a list of email clients (if you have more than one) to choose. If you only have one, that will be the default. Even if you have many email clients on your phone, if you only set up your accounts in one, let's say Outlook, that is the one that will open.
