Receipts from donationManager are sent using a defined Organization-Wide Email address.
To update the email sender follow these steps:
Go to Setup > "Organization-Wide Addresses" and make the required updates
If you're changing the name or the address entirely you'll need to update settings so fundraisingManager knows which address to use.
Go to Setup > Custom Settings > cloudStack Settings
Click Manage.
Then Edit the field Default Org-Wide Email Address Name
To learn more about Org-Wide email addresses - please see here