Universal Mailer is a Mail.app plugin that solves some issues when sending emails. It removes ATT00001.htm files, it correctly formats messages with attachments and inline images, and offers an option to set the default font for outgoing messages.
When enabled, Universal Mailer is able to correctly format outgoing emails, so that even after Exchange EWS filtering they retain the original format.
The plugin runs in the background without interfering with your work: all you have to do is set a default font for outgoing emails and write your emails as usual. When you hit the Send button, Universal Mailer checks the email for you and modifies it as needed.
Note: Now requires OS X 10.11 or later