To use Mixmax features like Templates, Meetings, Reminders, Tracking, and Smart Send when you compose a reply to an email, set Gmail's reading pane to No split or turn it off. To change this setting, see Google's help article on the Gmail reading pane.
How each reading pane setting affects Mixmax
Right of inbox
Emails open in a panel to the right of your inbox. When you compose a reply, a banner says "Mixmax currently does not support replies in preview pane mode," and the Mixmax buttons don't appear in the reply window.
Below inbox
Emails open in a panel below your inbox. You'll see the same banner when you compose a reply, and the Mixmax buttons don't appear in the reply window.
No split or reading pane turned off
Emails open in full view. When you compose a reply, the Mixmax toolbar with Meetings, Reminders, Templates, Sequences, and Collaborators appears above your message, and Smart Send and the Tracking icon appear next to the Send button.
FAQ
Why do I see "Mixmax currently does not support replies in preview pane mode"?
This banner appears when you compose a reply while Gmail's reading pane is set to Right of inbox or Below inbox. To use Mixmax features in your replies, set your reading pane to No split or turn it off.
Can I just close the banner?
Clicking the X hides it, but it comes back the next time you reply to an email. Changing your reading pane setting stops it from showing up.