If you are unsure about approaching this issue please call our helpdesk as you can irreversibly damage your PC if you are not careful
You can avoid “firstname.lastname@example.org [mailto:email@example.com] On Behalf Of” If you uncheck the box “Enable Sender ID compliance” in SFDC but the reason the compliance email settings is enabled because when you send an email from salesforce.com, SFDC spoofs our email address. Some companies do not accept unannounced spoofed emails because spammers will do the same thing, and puts security at risk.
Go to Setup | Administration Setup | Email Administration | Deliverability
Uncheck the "Enable Sender ID compliance" checkbox