In the sales module, if "each mail sent to the alia creates a new opportunity" is configured, incoming mails to the sales mailbox do generate opportunities. However, the sender does not get automatically subscribed to the conversation related to the opportunity. So, when the sales person writes back and does not check and invite the sender to the conversation, nobody will receive his message. Opportunities generated from the contact form however do automatically add the sender to the list of recipients. 

Is this behaviour deliberate and how can it be changed?