See Postfix Authenticated Smarthost

You may need to use TLS for modern SMTP servers.

FreePBX will maintain the credentials, but will not enable TLS.

FreePBX Console → Admin → System Admin → SMTP Email Setup

Preliminary - From FreePBX forums

One method for after-hours calls is to have a specific extension (or ring group) that the after-hours calls are directed to (via IVR, DID, time condition, etc). This extension has follow-me configured for the after hours tech's cell number (this accomplishes essentially forwarding the call to the tech cell) with “confirm calls” configured in follow-me (this accomplished the “press 1 to accept, etc). If the call is missed, we have the extension set to send it to a voicemail blast group that includes the after-hours tech and manager/supervisor's extensions. These extensions should already be configured for voicemail to email (this will accomplish the voicemail to email for the tech and anyone else that needs to see).