"Error Message-An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please verify that your email address is correct in your account settings and try again." I have two addresses(Same Domain). Address A works fine when sending a message. I get this error message when sending a message using address B. No problem receiving mail with either address. What do they mean by "Mailbox unavailable"? . I checked the outgoing server of address B, It is not added automatically so i was added manually, But still it is getting an error. Your help would be appreciated.