A good rule of thumb is to have 1 Replyify account per send-from address, but sometimes you need to use an alias or a separate SMTP server depending on your use case.
The steps to add an email alias are as follows:
1. Add an email to your account here: https://app.replyify.com/account/profile/emails
2. Confirm your email by clicking on the link in your inbox.
3. (A signature will automatically be created for you)
4. Edit the new signature
Pro tip: You do not have to have content for your signature, if you only want to change the send from address you can have an empty signature so that no extra content is added to your emails but the correct send-from address will be used
5. Make sure you have the correct SMTP permissions to send from this new email address. If you are using Gmail you can find this in the Gmail settings under 'Send mail as'