what's the deal with pre-existing packages?

[daniel]

i'm new to freebsd, formerly of gentoo and i'm a bit confused with the fact 
that sendmail is pre-installed when i did a minimal install -- especially 
when i find that there's 5 sendmail's in ports.  so i have a few questions:

 1. is the sendmail that's already there checked for security problems when i
    run "portaudit -a" or "portupgrade -a"?

 2. what's the difference between the other 5 sendmail's in ports?

 3. if i install a ports sendmail (/usr/ports/mail/sendmail), i then have
    two copies of it on my machine.  can/should i remove the default installed
    one?

 4. if i do install a ports sendmail, how do i get it to start at boot time?

thanks for your help

-- 
only after the last tree has been cut down
only after the last river has been poisoned
only after the last fish has been caught
only then will you learn that money can't be eaten.
  - unknown