To enable dma please edit /etc/mail/mailer.conf to replace all lines
referring to another MTA with the following:

sendmail	%%PREFIX%%/libexec/dma
send-mail	%%PREFIX%%/libexec/dma
mailq		%%PREFIX%%/libexec/dma

If you not need sendmail anymore, please add in your rc.conf:

sendmail_enable="NO"
sendmail_submit_enable="NO"
sendmail_outbound_enable="NO"
sendmail_msp_queue_enable="NO"

if you want anything in your queue to be flushed at on boot or before
shutdown, add the following to rc.conf as well:

dma_flushq_enable="YES"

you can also add a cronjob to do regular mailq flushes using the following 
command: 

%%PREFIX%%/libexec/dma -q

And you can disable some sendmail specific daily maintenance routines in your
/etc/periodic.conf file:

daily_clean_hoststat_enable="NO"
daily_status_mail_rejects_enable="NO"
daily_status_include_submit_mailq="NO"
daily_submit_queuerun="NO"