Question : smtp mail on sbd 2008

I have just set up my first sbs2008. The users and shares are active. I have pointed the ISP A record to the public IP of the server and got the remote web workplace running. Can someone confirm that all I need to do to get exchange working is to point the ISP MX records to the public IP of the server.
I realise  this can take 24-48 hours to propagate but is there any other DNS changes required?
Also I have been having problems with self certificates. There are only four people in this organization. What is the cheapest and easiest way to get a trusted certificate and how do I go about it.

Answer : smtp mail on sbd 2008

Ye all you need to do is point the MX record to the A record.
See my article here for guidance on DNS configuration: http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_2370-Exchange-DNS-Configuration.html

Also make sure you use the trusted certificate wizard in SBS as far as the certificate goes.  You will need a SAN/UCC certificate.

I also found a nifty little utility for managing certs with Exchange 2007 have a look at my blog post here: http://demazter.wordpress.com/2010/06/15/exchange-2007-ssl-certificates/