First check the local mail.box database and delete or send any messages in there.
Then check the local smtp.box database and delete or send any messages in there.
(This smtp.box is new to me and is to do with pop email, or sending email via a non notes smtp server )
NB: To open the databases do a File->Database->Open or CTRL+O
and then seelct server "Local" and then enter mail.box or smtp.box in the filename.
CAREFUL: Do not delete mail.box on a Server !