Have you requested that AWS open mail related ports to your instance?
This may be part of the very last step in that (slightly outdated) tutorial. Or it may be a separate step, I do not know what AWS’s specific requirements are.
I use LightSail on many of my servers. On new accounts port 25 is limited. You can request AWS to remove the restrictions. You have to fill out a form and inform that what you will be using your email server for and that you have spam policies in place. If you have a WordPress website I highly recommend the plugin Mailster.