When I send an Action Alert, the From field contains the organization's name and my name as "On Behalf Of".
Example: From: <organization's name>[mailto:email@domainName.org] On Behalf Of <alert sender>
When an alert is sent it always contains the organization as the sender. Outlook will display the sender information (organization), while other email services do not. The actual constituent's email address is always the reply to address, so if a representative replies to the email in all of the most common email clients (including Outlook), it will go to the constituent.