Question
The lead is created from the data gathered via landing page. The "Send message" process element is used to reply contact email.
Process Log indicates that the process has been completed.
But the Execution Diagram shows an issue:
Answer
The business process is using the mailbox is not marked as "shared mailbox", hence, only owner of mailbox can use it for sending. But business-process is running under Supervisor.
You need either to connect mailbox to Supervisor and use it, or set up the mailbox as shared one. You can find guide here:
https://academy.bpmonline.com/documents/studio/7-12/configuring-shared-mailbox