I am sending email through the Mandrill gateway. Site Default Email (on Site Information page): abc@xyz.com "E-mail from address" on the SMTP configuration page: abc@example.com When I send a test email, I see no error, but when I log into Mandrill and check the outbound emails, I saw that Mandrill rejected the email because it was sent by xyz.com, which it should have been sent by example.com. As a workaround, you can change the Site Default Email to match the "Email from address."