IOErrorEvent ID 2031, AIR desktop and local server

I have developed an AIR application for Windows desktop (AIR v22.0.0.153), which allows users to design a logo and then send it via email (using airxmail v0.7). I have tested the 'send email' functionality using a gmail account and SMTP, and this works fine on several PCs. The application is installed on a PC in a museum, which is connected to the local network there. The app installed at the museum can successfully send the emails from my testing gmail account, however, the intention is to send the emails from the museums own email account, via their own local server.

No authentication is required to send the emails through the server at the museum, I just have the details of the SMTP host and port. Whenever the app attempts to send an email using the server, it triggers IOErrorEvent with errorID 2031.

Can anyone advise what is required to make this work?