Gmail, Yahoo and AOL email address communications are sent from firstname.lastname@example.org due to DMARC restrictions (ie. TeamSideline can no longer send emails "on behalf of" for Gmail, Yahoo and AOL email addresses). The sending of emails is constrained due to the DMARC restrictions being put in place by these email providers to help protect email senders and receivers.
Here is some more information on DMARC restrictions - http://www.dmarc.org/.
The "Reply To" email address for all TeamSideline messages is still set to your email address so when your customers reply back to the email, the replies should be sent back to the sender (ie. your email address).
If you would like, you can create and use a domain based email address instead as your "From" email address.