What are you trying to accomplish? Receive mail using your DynDNS address? Does your ISP block port 25? Are you trying to send mail via your ISP smarthost?
If your only trying to send mail via your ISP smarthost, what I did for a simple home setup was add:
to /usr/share/sendmail/cf/openbsd-localhost.mc file. Then built the .cf file.
# m4 ../m4/cf.m4 openbsd-localhost.mc > openbsd-localhost.cf
# cp openbsd-localhost.cf /etc/mail/localhost.cf
Then restart the sendmail process or restart the system. There's probably other methods and network tweaks such as having sendmail listen on other than the localhost.
Make sure you edit your /etc/rc.conf.local file to reflect what you called your .cf file in /etc/mail.
sendmail_flags="-L sm-mta -C/etc/mail/localhost.cf -bd -q30m"