MailKit SMTP送信エラー

SmtpClient cli = new SmtpClient();
await cli.ConnectAsync("smtp.boo.foo",25);
await cli.SendAsync(msg);

An error occurred while attempting to establish an SSL or TLS connection.
The host name (xxx.xxx.xxx) did not match the name given in the server's SSL certificate (yyy.yyy.yyy).

await cli.ConnectAsync("smtp.boo.foo",25,SecureSocketOptions.None);

# Check the /usr/share/doc/sendmail/README.cf file for a description
# of the format of this file. (search for access_db in that file)
# The /usr/share/doc/sendmail/README.cf is part of the sendmail-doc
# package.
#
# If you want to use AuthInfo with "M:PLAIN LOGIN", make sure to have the
# cyrus-sasl-plain package installed.
#
# By default we allow relaying from localhost...
Srv_Features:   S
・・・