The error "0x800CCC78" indicates Unknown sender. This is caused by having the incorrect e-mail address in the Reply-To field.
Verify the address you have entered in the From field is correct. Make sure there are no leading or ending spaces or other simple to overlook typos in your address. Verify sending authentication is correct in the Outgoing server tab of the More Settings dialog
please reply back for more clarification and help.
Thank you :)