Delete a contact
First, delete your contact in Google Contacts. Mixmax syncs with your Google contacts every 15 minutes, so Mixmax will “learn” that you've deleted the contact. (Note that Mixmax does not sync your contact groups after you first create your Mixmax account.)
To delete your contact in Google Contacts, go here: https://contacts.google.com/
Then find the contact you want to delete. You may need to look in several of your contact folders.
Hover over the email and click on the three dots on the far right to access the more actions button:
The select delete: