I'm building skills for an entry-level IT support role and have been practicing networking, ticketing systems, Microsoft 365 administration, and Active Directory. For my lab, I created a multi-branch organization with departments, computers, users, and GPOs.
I'm now learning hMailServer on a Windows Server 2022 virtual machine. I created two internal accounts, [email protected] and [email protected], and I'm trying to send mail between them using Thunderbird on a separate Windows 11 VM. hMailServer is configured to use IMAP on port 143 and SMTP on port 25, but the messages are failing.
What should I check first? I'm especially interested in SMTP connectivity, firewall rules, DNS or MX records, server bindings, and whether using a .local domain is appropriate for an internal lab. Is there another good free mail server application for practicing at home?
5 Answers
Start by testing the server locally before troubleshooting Thunderbird. Send a message between the two accounts from the hMailServer console or directly on the mail server. If that works, test connectivity from the Windows 11 VM to the Server 2022 VM, such as confirming that port 25 is reachable. Only after those tests pass should you verify the Thunderbird account and SMTP settings.
Check the basics in hMailServer: make sure SMTP is bound to the correct server IP, port 25 is listening, and the Windows firewall allows inbound SMTP and IMAP traffic. Also confirm that the client is pointing to the mail server’s actual IP address rather than an incorrect DNS entry. A temporary firewall test can help isolate the cause, but re-enable the firewall and add a proper rule afterward.
Keep in mind that hosting mail directly on the Internet is much harder than sending mail inside a lab. Public providers may reject messages from residential IP addresses, and proper DNS, reverse DNS, authentication, TLS, and spam protections are required. For practice, an isolated internal domain is a safer approach than trying to deliver mail externally.
The problem was resolved by checking the configuration step by step, including connectivity, DNS, SMTP and IMAP settings, and firewall access. After correcting the relevant settings, mail could be sent back and forth between both internal accounts. hMailServer is perfectly usable for learning; the troubleshooting process is useful practice in itself.
Verify the internal DNS records and the domain configuration. A .local domain is fine for an isolated home lab, but it is not a publicly registered mail domain and should not be used for Internet-facing email. Make sure the mail server’s hostname resolves correctly and that any required MX record points to it. For an internal-only setup, testing with local DNS and direct hostnames may be enough.
For SMTP, the destination server receives connections on port 25, while the client’s source port is usually assigned dynamically. The important part is allowing inbound port 25 on the mail server and making sure the service is listening there.