Send and Archive
You can turn on Send and Archive in your Gmail settings, and Mixmax will follow suit.
Open your Gmail settings by clicking the gear icon in the top right corner of Gmail and clicking Settings.
Scroll until you see Send and Archive, and click to select Show “Send & Archive” button in reply
Scroll down to the bottom of your settings page and click Save Changes.
If you are using the Mixmax compose window rather than the Gmail compose window, then you will also need to check your Mixmax compose settings to ensure this setting matches Gmail. Under Composing scroll down to Send and archive
You will now see the Send + Archive button at the bottom of the Mixmax compose window:
Note that you will only see that button when you are composing a reply. You won't see it when you're composing a new message, because there is nothing to archive.