sendmail binds only for localhost, you need to change that.
search for the line which is making sendmail bind to 127.0.0.1 (in vi just do /127.0.0.1 ), you need to add dnl to that line and then update your sendmail conf file.
m4 sendmail.mc > sendmail.cf
service sendmail restart
and in /etc/dovecot.conf, there is a line where it says the protocols you are serving, the line is hashed, remove the hash and restart dovecot.