Outbound messages are available as a core action in Flow Builder. Send a SOAP message from a record-triggered flow to a designated endpoint. ... Perhaps you used outbound message actions with workflow rules and approval processes.

Can flows send outbound messages in Salesforce?

Though it can't send an outbound message like the workflow, with process builder you can update any related record, send an email, invoke a flow, submit a record for approval, and post to chatter.

Can approval process send an outbound message?

You can send the outbound message using workflow, entitlement processes, or approval processes. The best thing about the outbound message is, you do not need to write any apex code you can create it by simple point & Click.