Mago allows you to send email from multiple email addresses, not just the one you used when you signed up. This is useful if you offer a variety of services, each with different branding or if some client emails are sent from an assistant's email address.
If you wish to change the "from" email address when manually composing an email, simply use the drop-down list next to "From" (see image)
If the email address you wish to use isn't on the list, simply click the "+" icon to the right of the dropdown list to add an additional email.
If you are using automation to send an email, the "From" address is defined for the automation routine itself.