By default all emails sent to your customers are showing as sent via FieldInsight showing the sender's name.

To customise the email sender's name, open settings misc and in the Email messages section change the Email full name - use the {sender} placeholder to show the name of the sender (the scheduler or technicians that send the email).

Please note that this will change the display name only, not the reply to address.

To change the reply to address for sales documents, select a new email address in the Reply to email address for sales documents located further down in the Sales document section