You cannot use all of the third party fonts with Apple Mail yet. However, you can still use the app to get an access to the iOS font manager. It is just a matter of time that you will be able to use a wide range of third party fonts for the Apple Mail as well.

• There is an angle bracket that the top right side of the keyboard. Tap it to get access to the new quick toolbar.
• You can view available formatting options by tapping on the “Aa” icon.
• Tapping on the Default Font will bring up the font manager. That font manager holds all of the correct fonts. The new fonts that you download are also going to show up in this list. Although you may be able to see them at this time, these fonts may not load when you try using them in certain apps.
• You can go to the list of installed fonts by going to Settings > General > Fonts. From there, you can choose to be prompted to App store to download new fonts to download.
• You can tap on the Open App Store button and tap search icon. You can enter the word Fonts. There are going to be a number of categories to choose from.
• You can tap on any of the font files to view extended details about the font. You can also view letter/number, paragraph, and character formats.

Right now, there are very few options for the free fonts available. However, you can hope for the new custom fonts to be available over time.