Send email using an alias address?

I have an email forward from another organization, with email address OtherEmail.org. That means, if someone sends email to OtherEmail.org, it is forwarded to my account on Comcast.

When I send email from Comcast, I want it to appear as if it is coming from OtherEmail.org, so when recipients hit Reply, the reply goes to OtherEmail.org. How can I do do this?