1) If you are permanently removing qmail, you should also 'rm -rf /var/qmail' and remove the qmail users/groups.

2) Do not forget to reconfigure your /etc/mail/mailer.conf to use your older MTA, or reconfigure your old MTA binaries if you decided to replace them.

3) Do not forget to enable sendmail as your default MTA if you have to. You can do it just executing 'echo sendmail_enable="YES"' >> /etc/rc.conf