In pfSense navigate to Sytstem > Advanced > Notifications

E-Mail Section

E-Mail server:

SMTP: 465

Secure SMTP Connection: Check this box

Validate SSL/TLS: Check this box

From e-mail address: The address you want in the from field.

Notification E-Mail Address: The address you want the email sent to.

Notification E-Mail auth username: Your Fastmail main email for login

Notification E-Mail auth password: The temp password created for this task

Notification E-Mail auth mechanism: PLAIN

Test the settings. If the emails come through the way you want click save at the bottom.