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The SMTP client uses TCP Port 25 because that's the standard port for SMTP and has been since the beginning of the Internet.
The problem, if course, is that ordinary e-mail has been abused by spammers to the point where many ISPs block outgoing Port 25 mail, and most Internet-facing mail servers won't accept incoming Port 25 mail.
The little CPU in the MicroLogix 1400 doesn't have the horsepower to do encryption, and most applications where it's sending e-mail will be done with internal SMTP servers on a corporate network that can be set up to accept Port 25 mail from the MicroLogix specifically.
Another category of devices that suffers from the same problem is the first generations of all-in-one copier/printer/scanners that have a built in mail client. So there's a market for ways around the problem.
For systems that need to send mail to a destination outside the network you have control over, you generally have to set up an SMTP Relay that will forward your ordinary Port 25 e-mail via the more sophisticated authentication methods used on the modern Internet like SSL.
hMailServer is one that I've read about having good results.
I haven't personally done any work with the MicroLogix e-mail client outside of setting up internal implementations using the ArGo Software mail server.