When you send an email using an alias address, Mixmax allows you to continue to use your alias once your recipient replies. Here are the steps to take if you always want to reply from the alias the email was sent to:

The default reply controls are in your Gmail settings. From Gmail, first, click the Settings (gear) icon in the upper-right corner of the page:

Next, click the Accounts and Import tab and make the change under the When replying to a message setting. After that, click the Import Email Addresses to Mixmax button, and you'll be all set!