SME server by default will block any outbound connections from your LAN workstations attempting to connect to an external SMTP server on port 25. You can check your setting for this in server-manager under 'Security' / 'Proxy settings' / SMTP proxy status (the default setting is 'Blocked').
If your SME is blocking outbound SMTP traffic, the outbound SPAM is either being relayed through your SME server or its being sent out on another common SMTP port (465 or 587).
You can look for outbound smtp traffic as described here:
Mail log file analysis#qmail: Outgoing SMTP trafficYou can monitor traffic passing through your SME server in real-time using 'iptraf'. There's no particularly useful wiki page on iptraf, but there are some forum discussions that might help:
Google search for iptrafIf you find that one of your workstations is generating lots of traffic to outbound systems on port 465 (SMTPS), you may be able to block that in your SME server - there are some old notes about using custom template fragments to block outgoing traffic (these are very old notes; proceed with caution):
https://wiki.contribs.org/Firewall#Block_outgoing_portsIf your LAN switches provide traffic monitors you might also figure out which workstation is causing problems by looking for unexpectedly high traffic there.
There could be nothing wrong with your workstations or network - many DNSBL services block IP addresses that they consider to be dynamic (IPs intended for home users). Or, if your ISP has 3 or more infected (or spamming) clients on the same class C subnet, your entire subnet could be blocked (including your IP). You can research which block lists are blocking your mail and why using the
MX Toolbox Blacklist Tool.
Another possibility -
Some email providers will not accept your email if the name returned by the 'reverse lookup' of your IP address does not in its turn result in your IP address when it, itself is looked up. It doesn't necessarily need to match your configured domain name, but it has to work both ways.
For example, if the nslookup d.c.b.a.in-addr.arpa (the reverse lookup for your IP) returns dsl-a-b-c-d.mycity.myispsname.com then before trying to send email directly from your SME to the Internet at large you want to make sure that nslookup dsl-a-b-c-d.mycity.myispsname.com returns your a.b.c.d and not an error or some other address.