Options For Receiving Email in SBS 2003 and SBS 2011
If you want to feed email directly to an SBS server using SMTP, perhaps from a spam filtering service or similar, then port 25 also needs to be open in addition - but take care to authorise only sending server IP addresses that need to be authorised.
Information on securing SMTP in Exchange 2007 can be found here (the process is nearly identical in Exchange 2010): Configuring Exchange 2007 Hub Transport role to receive Internet mail
As an alternative, you can configure an ISP to hold email in a POP account and then pull down email to the server using PopBeamer. A disadvantage of this approach is a small delay in receiving emails (PopBeamer can be configured to check for mail every minute, so not a long delay). An advantage though is that port 25 can remain closed - so there is one less point of entry for a hacker to gain entry to your network with.
More info here: POPBeamer for Microsoft Exchange