newburns
Anything connected to the Internet is open to attack.
Your system remains secure by using secure connections, such as SSL, by having strong passwords & implementing wise system practices, such as keeping all aspects of your server & firewall up to date with regular bug fixes & software upgrades, especially for all web based applications running on your server. Watch out especially for cross application vulnerabilities
By having less security you increase the vulnerability of your system & make it easier for attacks to occur.
If port 25 is open on your firewall & forwaded to sme server, then hackers have a direct connection via port 25.
Whether they can make use of that, depends on the security of your whole system, including email passwords etc.
Think carefully before you do what you are saying as you have really introduced a vulnerability.
I am already listed on the CBL Spam list for HELO. Not sure what that means, but I can't seem to get off of that list.
Perhaps an indication that configuration of your sending server does not meet stricter external mail servers guidelines, read the sme server Manual Appendix (re PTR & SPF), & perhaps you should be sending mail via your ISP's smtp server, if you cannot ensure the integrity of your own mail server.