When this occurs, it generally indicates that your outgoing e-mail was set incorrectly. You may have either deleted your default identity, or you modified it and cleared the "From: address". To fix this, login to webmail and click the "Options" icon. Then click the link that reads "Identities / Email Addresses / Signatures." Edit your identities and make sure that you type the correct address in the "Your From: Address" field and "Your Reply- to: Address" field.