The problem is not about the font the app uses, because the app DOESN'T SPECIFY A FONT. This means that the font used for display is whichever font has been set as default in the client where the email is received.
Outlook for Android uses a <body> tag for normal text and gives you two other options - subhead and Title. All either of those does is to add a bold attribute to the text. You can check this by sending yourself an email and looking at the raw HTML of the message.
People report seeing different things because they are reading the emails using different clients, or clients with different default fonts set.
As an example, here is what the Outlook client on Windows does:
To Change the Font on Incoming Messages (slipstick.com)
Hope this helps