Just a quick follow up on this for anyone else reading! We were able to get this resolved, and believe the Send From button wasn't displaying due to cached web interface components. If you run into these issues in the future please try the following:
1. First, you need to adjust the configuration of the desired alias so that sending from that alias is allowed. Attached you will find a screenshot that shows the settings toggle that should be enabled to allow domain users to send through the alias account. These settings are found within Domain Settings>Accounts>Aliases. Once this toggle is enabled, all domain users who are included in the alias should see the alias address as an option in the SEND FROM dropdown when sending email.
2. Once the alias configuration has been adjusted users only need to select the Send From drop-down menu in webmail to choose this alias account when sending emails.
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com