The email subject can be customized per document template.
The default email subject is:
"User XYZ sent you ABC #N using megaventory.com"
Where XYZ is the user that emailed the order/document and ABC is the document template type (sales order, purchase order, invoice etc.) and N is the document number.
To customize the subject, navigate to the list of Document Templates and choose the template type to change this for (for example Sales Order):
Then, tick the respective box and Edit:
One can use special text handlers to fetch info and place the info in the subject line:
This will translate to "You are receiving this email from ACME Inc.", provided your business name is ACME Inc.