This only affects web-based email using a browser (Chrome, Firefox, Safari). Computer-based programs will need to be setup according to its documentation.
Step 1: With Mail open, click on the settings icon in the upper right.
Step 2: Click on "See all settings."
Step 3: Click on the "Accounts" tab.
Step 4: Click on "Add another email address."
Step 5: Fill in your @gommunitas.org alias according to the following convention: firstname.lastname@gocommunitas.org. Make sure the "Treat as alias" box is checked and click on "Next step."
Step 6: Click on "make default."
Step 7: You are now all set up. Whenever you send an email from your browser-based Mail account, it will your new @gocommunitas.org alias. Congratulations!