I am not so sure I agree with the hard drive storing network settings ...
This may sound over simplified, but if you will notice in the screen shot you posted the first line says "selecting 'save' in the main window is required to save the new settings". My question would be, are you simply clicking 'OK' then exiting TopAccess, or are you clicking the SAVE button first?
If you are, indeed, saving the setting properly, then I would ask if you have tried using an explicit IP address for the SMTP server. Just a thought, that one, because I do not think you would loose the settings even if the server was not found ... but I am just throwing out ideas.