sendmail

i want to make my web server, mail server, authentication server separately. Is this possible??? i’m using red hat linux and sendmail 8.9.3-20. my concern is that, if i make my mail server separately, do my users still get authenticated when they check emails? do i need to copy from my authentication server, passwd and shadow to my other servers??? where can i find instructions on this problem??? any help would be greatly appreciated. thanks.