Download PostCast Server Free Edition.
Install.
Run and access Settings.
Hostname should be your computer name or IP address.
Server Port should be 25.
DNS server... well, it may not be needed at the moment.
Press OK and don't close the window.
Check Email Options in ZenCart's Configuration.
Code:
E-Mail Transport Method smtp
SMTP Email Account Mailbox (leave blank)
SMTP Email Account Password (leave blank)
SMTP Email Mail Host (your computer name or IP address)
SMTP Email Mail Server Port 25
Do your normal ZenCart activity involving sending emails.
The emails will be delivered there, no matter the sender and receiver.
That way, you can also know where they exactly go.
And view the contents, of course.
If you want to continue to send them to their respective recipients, just press Start in the PostCast window.