I received an email from the Gmail team saying an email sent from firstname.lastname@example.org (to email@example.com) has been blocked because it contains a virus. Do I have to worry that someone has gain access to my accounts and it really able to send email from mydomain? Thanks for the help.
Not necessarily. https://en.wikipedia.org/wiki/Email_spoofing
Thank you very much! You have put my mind at ease.