Last month, a reader, Aaron Watson, said that it would be possible due to SMTP authentication. Thanks to his message, I had a new way to continue finding out why my hMailServer was not working.
First of all, I set a new IP range record in hMailServer with both Lower IP and Upper IP being the same as my Windows Private IPv4 Address.
Next, I unchecked one of the checkboxes which says “External to external email addresses” under the “Require SMTP authentication”.
Finally, hMailServer should have something like this as shown in the following screenshot.
With this step done, I can now successfully send email via hMailServer on my laptop. My laptop is also a SMTP server now. =D