Default MTA is set to the SMTP email address I've setup.
As for the button, if you go to the end-user portal and click Check Ticket Status it brings up a form where you enter the email and ticket number then click "Email Access Link". Clicking that button will generate two errors:
Error 1:
Unable to email via SMTP:smtpserveraddress:587 [email@domain.com] Failed to add recipient: @localhost [SMTP: Invalid response code received from server (code: 501, response: 5.1.3 Invalid address)]
Error 2:
Unable to email via php mail function: mail() returned failure
On error 1, I have no idea why its trying to add @localhost as a recipient. On error 2, I can't figure out why its using php. All other alerts and notifications go out just fine. On the ticket I was just testing the end user receives the notice a ticket has been created, agents get the alert, users receive updates from agents with no problem. I thought it might be non-ticket related emails, but I can send password reset links to agents with no issues.