When a receipt email is automatically sent to your donors in response to an online gift, it is sent from email@example.com.
This won't impact the deliverability of your receipts, but you may want the additional customization so your donors see someone more familiar.
Change the "from" email address
To allow for this to work, you will need to add two text records to your DNS settings.
Follow the instructions here, and add these two records (SPF and DKIM) on your domain.
Once you're done, let us know at firstname.lastname@example.org and we can monitor the status!
Change the "reply to" email address
Alternatively, you can leave the email's "from" field to remain as email@example.com and just change the "reply to" field of the email. This will allow any reply to that email to be sent to an address of your choosing.
Go to Settings, then Basic Settings. At the bottom of that list you'll see Set your "reply to" email address where you can fill in the address you'd like.