Trying to create my own SMTP server

Question

I am trying to setup my own SMTP server on my local NAS and when I send an email to my outlook or hotmail accout I get

connect to hotmail-com.olc.outlook.com:25: connection timeout

Thank you for you answers in advance.

Answer 1

This is a very common issue. I am quite sure that this is due to port 25 being blocked by your ISP.

ISPs commonly block outbound port 25 (SMTP) on residential and small business connections to prevent spam. This is an industry-wide practice and has nothing to do with your NAS or mail server configuration.

You might want to reach out to your ISP to confirm. Some ISPs will do this on request, especially for business accounts. Residential accounts are usually a hard no.

Answer 2

https://www.reddit.com/r/microsoft/comments/1eabwa9/comment/lsi6x9c/

Microsoft made changes to its email services on September 16th, 2024. They ended support for SMTP on free accounts [using Basic Authentication], which means users with free Microsoft accounts can no longer use SMTP to send emails through third-party apps or devices. This change primarily affects users who rely on email clients or devices that use SMTP for sending emails.

https://learn.microsoft.com/en-us/answers/questions/4709884/outlook-problems-from-september-16-2024

This means you need to make sure your third-party email app supports modern authentication, such as OAuth2.

Server Settings

https://support.microsoft.com/en-us/office/pop-imap-and-smtp-settings-for-outlook-com-d088b986-291d-42b8-9564-9c414e2aa040

Answer 3

The connection timeout indicates that the SMTP server on the NAS cannot successfully reach Outlook.com/Hotmail over port 25. Troubleshooting should focus on basic SMTP connectivity and DNS.

Follow these steps:

1. Test direct SMTP connectivity to Outlook.com
   • From the NAS (or another machine on the same network), run:
     • nslookup -q=mx hotmail.com
       • This returns the current Outlook.com MX hosts (for example, mx1.hotmail.com, mx2.hotmail.com, mx3.hotmail.com, mx4.hotmail.com).
     • Then test each MX host with telnet on port 25, for example:
       • telnet mx1.hotmail.com 25
   • If the connection to some hosts times out, test all published IPs, because occasionally some IPs in the MX record may be out of service.
   • If connecting directly to the IP works but connecting to the hostname does not, there is likely a DNS issue on the NAS or local network DNS server.
2. Verify that outbound port 25 is allowed
   • If all telnet tests to Outlook.com MX hosts on port 25 time out, check:
     • Router/firewall rules blocking outbound TCP 25.
     • ISP restrictions on outbound SMTP (many ISPs block port 25 from residential connections).
   • If port 25 is blocked, either:
     • Request unblocking from the ISP, or
     • Use a smart host/SMTP relay that listens on another port (for example, 587) and configure the NAS SMTP server to relay through it.
3. Ensure the NAS SMTP server is correctly configured
   • Confirm that the NAS advertises a public, routable IP and not a private IP (such as 192.168.x.x, 10.x.x.x, or 172.16–172.31). Outlook.com may not accept email from senders that fail reverse-DNS lookup or advertise non-routable IPs.
   • Configure proper DNS and reverse-DNS (PTR) for the public IP if possible, so the HELO/EHLO and reverse lookup match a valid hostname.
4. Test the local SMTP server itself
   • From a machine that can reach the NAS, test the NAS SMTP server with telnet:
     • telnet <nas-hostname-or-ip> 25
     • Run a basic SMTP conversation (EHLO, MAIL FROM, RCPT TO, DATA) to verify the NAS accepts and queues mail correctly.
   • If messages stay queued on the NAS, inspect its SMTP logs/queue to see the exact error when it tries to deliver to Outlook.com.

If after these steps the NAS still cannot connect to Outlook.com on port 25, the most common causes are ISP port 25 blocking or DNS issues. Using a reputable SMTP relay (smart host) instead of direct delivery is often necessary on home/consumer connections.

---

References:

• Sender Support in Outlook.com
• Appendix D: Create the SMTP Server