Sendmail still using port 25

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

Output :

tcp        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 :

pkill sendmail