Автоматизированные письма из документации относительно закрытой сети через adsl на пользовательские электронные письма

If you limit SMTP to allow outbound only, then no, there are no security concerns as far as an unsolicited inbound connection exploiting an unpatched / vulnerable system.

Without knowing what your firewall is, I will go out on a fairly sturdy limb and say "yes", you can create a firewall rule that only allows port 25 inbound communication to go to the Exchange server. That is in fact a common thing to do to secure email communication and prevent inbound / outbound SMTP tomfoolery. In fact, if your systems are closed and predictable enough, you can create firewall rules that only allow outbound / inbound email to and from the two email server systems. Of course, this gets tricky depending on how email clients access the server and if they too are expected to communicate through email, but a deeper discussion of that kind of network design will require more information about your specific network.