Whereas you have uninstalled it, because you have a new mail system (like PostFix or Exim4).
Try again to uninstall it and its dependances :
apt-get purge sendmail-bin sendmail-base opensmtpd
Check port 25 :
netstat -lnp | grep :25
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 28817/sendmail: MTA
It’s still there.
Locate Sendmail :
find / -name "sendmail"
Try to restart Sendmail to know if the package is still active :
service sendmail restart
Ultimate solution :