Where: Settings → Company Sending.
How email is sent: three tiers, tried in order
Connecting a mailbox (OAuth)
Connect a Gmail or Outlook mailbox in Settings → Email Connections. Once connected, it can send from that address for personal CRM email and, optionally, as the designated tenant transactional sender.
Send-only by design
The OAuth scopes are send-only. PortaPro can send from the mailbox but cannot read it. That's what avoids the Google CASA and Microsoft 365 certification hurdle. Reply capture is handled by the Reply-To and BCC mechanism, not by reading your inbox.
Reply-To on connected-mailbox sends
When Email Inbox Capture is on, conversation emails you send through a connected mailbox set Reply-To to your inbox address and get auto-BCC'd to it. Redundant capture, so replies land in the Inbox even when the reply is unusual.