In Airmail, there are three types of text editor which are - Rich Editor, Plain Text and Markdown. Changing the type of composer will just affect Airmail and will not make any changes system-wide. However, you’ll need to make changes 'per account' in your preference option which will allow you to carry out your own composer style for a particular account.
In the following screenshot there is an option that allows you to change the type of the composer:
Setting this option will only affect for the selected account, however, if you’re willing to change for entire composer we suggest you to change them for per account since because we do not have an option which is supported to change for the entire Airmail app at once.