To protect against the use of Action Network to send out spam, abusive, or malicious email, Action Network requires users to verify that they own their reply-to email. This is the email address activist's can reply to after receiving one of your emails. It can be linked to a domain you own or a gmail, yahoo, etc. You will not be able to send emails from a custom reply-to unless you add it here.
To verify your reply-to, go to the Settings tab in the Group Manage page and click Email Settings. If you do not have a group, you can create one for free. Click Add New Email Address and enter the email address you'd like to use as your reply-to. You'll receive an email where you can click to verify you own the email address being added. You can add as many reply-to emails as you need, with similar verification steps.
You can set a default reply-to that will appear on all emails created by your group automatically. If you want to use a different reply to, click edit on the Edit Email page and a drop down menu will appear where you can select a different verified reply-to email.