Please follow below steps and screenshots.
1- Open SafeSend Returns
2- Go to "Settings"
3- Client Instructions
4- To the right of "Initial Email"
5- Click "Edit Instructions"
6- In the subject line add any variables or make any changes you would like. A common variable is <TAXPAYERNAME>
7- Click "Update"
This example will now show the Taxpayer name for individual returns and company name for entity returns in the subject line of the email they receive.