Yes, if you can test it all out in a VM. Also keep frequent backups of your .conf files.
The main log file of interest will be /var/log/mail.log check that everytime you change something as many errors that are silent elsewhere will end up here.
It would be quite difficult to write a decent guide in a short space (also I have not quite got my setup correct yet), but make sure you RTFM before you start.
Depending on how many users you have, if you are using postfix it may be easier to just map the users mail accounts to unix accounts rather than use virtual mailhosts (these allow you to designate a single account to store mail which will be divided into seperate folders for different domains/accounts, so useful if you are running many domains/users as you don't have to update main.cnf and change unix settings for each account) .
here is a basic guide (assuming you are not using virtual mailhosts):
It would be quite difficult to write a decent guide in a short space (also I have not quite got my setup correct yet), but make sure you RTFM before you start.
Depending on how many users you have, if you are using postfix it may be easier to just map the users mail accounts to unix accounts rather than use virtual mailhosts (these allow you to designate a single account to store mail which will be divided into seperate folders for different domains/accounts, so useful if you are running many domains/users as you don't have to update main.cnf and change unix settings for each account) .
here is a basic guide (assuming you are not using virtual mailhosts):
http://rimuhosting.com/support/settingupemail.jsp?mta=postfi...
and one on integrating spamassasin:
http://www.akadia.com/services/postfix_spamassassin.html
Of course make extra sure you are not running an open SMTP relay (best way to check for sure is with telnet)!
Also you can setup fail2ban to watch your mail server logs for failed logins and automatically update your firewall rules to keep them out.