sendmail question?

[indy317]

i configure a sendmail but i don't understand this
"
dnl # Uncomment and edit the following line if your outgoing mail needs to
dnl # be sent out through an external mail server:
dnl #
dnl define(`SMART_HOST',`smtp.your.provider')
dnl #
"
what is 'smart_host'?
what do i put in 'smtp.your.provider'?

how do i find out my 'smtp provider'?

i use a road runner cable. anyone please help me ? thankz you