Geoffrey might quite well be right about that downgrading: your smtp server seems to be running, however is not reachable from the outside world and you are unable to reach your isp's smtp server. All this used to work before the upgrade.
Nevertheless a few things:
Does SME 6.5 include the fetchmail contrib (listed as Mails retrieval in your server manager)? There you can enable / disable direct smtp connections.
If you give the following command on the command line, what does it return: 'iptables -L | grep smtp' ?
If the line begins with 'ACCEPT', then the SME firewall is configured to accept smtp connections, however if the line begins with 'denylog', then smtp connections are blocked by the firewall.
And last but not least (as stated before): do make sure that your isp is not blocking smtp access to your server. My previous isp did so from one day to another without any announcement...