Email deliverability increases if the emails you send contain a plain text version of an email
MBR CRM supports this feature by default, and will actually automatically generate plain-text versions of the emails you send and attach those with the HTML email.
Use a Custom Plain Text E-mail
If you do not want to use the generated plain-text version, you can create a custom one within the email editor.
Select the Use Custom Alt-Body option and update the email. When the page reloads an area to edit the alt-body will appear below the email content editor.
If you are using the advanced email editor you can edit the plain-text version from the header bar.