How to find SMTP IP?

Finding your SMTP server’s IP address can be important for properly configuring email clients or troubleshooting email delivery issues. Here is a comprehensive guide on locating your SMTP IP address using various methods.

How to find SMTP IP?

Why You Might Need Your SMTP Server’s IP

There are several reasons why you may need to find your SMTP server’s IP address:

  • To configure email client apps like Outlook or Thunderbird for sending messages through your SMTP server instead of the default ISP server. This typically improves deliverability.
  • To add your SMTP server’s IP address to firewall allowlists so that emails are not blocked.
  • To troubleshoot email delivery problems by testing connections to the SMTP IP.
  • To route emails through a secondary “smart host” SMTP relay for better deliverability or analytics.
  • To identify unauthorized/unexpected connections to an SMTP server in server or firewall logs.

So if you are setting up email clients, managing email infrastructure, or diagnosing mail flow issues, you will likely need to track down that SMTP IP address at some point.

Locating SMTP Server IP in Email Clients

If you already have an email client like Outlook or Apple Mail configured to send messages through a specific SMTP server, you can view the IP address in the app’s settings:

Outlook SMTP IP

  1. Open Outlook’s settings menu.
  2. Click on “View or change existing email accounts” under “Email” in the left sidebar.
  3. Select your email account then click “Change”.
  4. Go to the “Outgoing server” tab.
  5. Look for the “Outgoing server (SMTP)” field – this shows your SMTP server’s hostname and IP address.

Thunderbird SMTP IP

  1. Click the Thunderbird menu button > Preferences
  2. Open the Accounts Settings section
  3. Find your email account in the list and click “Server Settings”
  4. The SMTP hostname and IP address are shown under “Outgoing Server (SMTP)”

So if you already have an SMTP server configured in your email app, just check there to find the IP address.

Checking SMTP MX Records in DNS

If you have not set up an email client yet or just need to verify the correct target SMTP server/IP for an email domain, you can lookup the MX records.

An MX (mail exchange) record maps a domain name to the server responsible for accepting emails sent to addresses @ that domain.

Here are the steps to find your domain’s SMTP IP from an MX lookup:

  1. On Windows run “Command Prompt”, on Mac open Terminal.
  2. Type nslookup then hit Enter.
  3. Enter set type=MX to check MX records.
  4. Run the command using your actual domain.

The results will list hostname records for the SMTP mail server(s) handling emails for that domain.

You can then do an additional DNS lookup on the SMTP hostname to find the server’s IP address:

  1. In the nslookup prompt, run: using your actual SMTP hostname.
  2. The resulting IP address is what you want for your SMTP server configuration.

There may be multiple MX records and SMTP servers listed for redundancy. Use the record with the lowest priority number first.

Consulting Email Admins or IT Staff

If you have access to the administrator or IT staff responsible for managing email services and infrastructure for your organization:

  • Ask them directly for the fully qualified domain name (FQDN) and IP address used for sending outbound emails via SMTP.
  • They can check DNS records or email server configurations to identify the proper IP to use.

This information should be readily available from any IT team managing enterprise email servers.

Checking Web Server Hosting Provider

If your email is set up through a hosting provider or IT services company, you can check there too.

Most shared web hosting plans come with SMTP servers included for sending email from websites. The hosting provider should indicate in the control panel, support site, or admin docs:

  • The correct SMTP server name to use like
  • The IP addresses of that SMTP server

This allows configuring SMTP credentials in email clients registered under hosting plan accounts.

Using Telnet for SMTP Server Discovery

You can also use the telnet command in command prompt or terminal to try connecting to port 25 of different systems on your network to see if an SMTP server responds.

Run telnet then the IP and port 25 to test various IPs until you get an SMTP service banner:

telnet 25


Connected to

Escape character is ‘^]’.  

220 ESMTP My Email Server

When you connect, the service name and 220 response code indicates an SMTP server is listening on that IP.

Repeat trying different IP addresses associated with your network until you discover the SMTP system. Make sure port 25 outbound is not blocked by your firewall.

Related Email Terminology

Here are some related terms when dealing with SMTP servers:

  • SMTP – Simple Mail Transfer Protocol used for sending outbound emails
  • SMTPS – SMTP over implicit or explicit TLS encryption
  • MX – Mail Exchange DNS record mapping a domain name to an email server
  • CNAME – Canonical Name DNS record aliases a hostname to an A record
  • A Record – DNS record that resolves a hostname to an IPv4 address
  • AAAA – DNS record used to resolve hostnames to IPv6 addresses
  • PTR – Reverse DNS record that maps IP addresses to hostnames

Key Takeaways on Finding SMTP Server Addresses

To recap the key techniques covered for locating SMTP IP addresses:

  • Check email client configurations (Outlook, Thunderbird, etc.)
  • Look up MX records and SMTP hostnames in DNS
  • Consult hosting providers, IT teams, or email administrators
  • Use telnet to discover responsive SMTP servers
  • Reference related DNS terms like MX, A, AAAA, CNAME, PTR records

Knowing these discovery methods can help you track down that pesky SMTP IP when needed.


Locating your SMTP server’s IP address is necessary for properly setting up email clients and troubleshooting mail flow issues. By checking existing email account settings, DNS records, server configurations, or using Telnet tests you can find the relevant SMTP IP.

Reaching out to hosting companies, ISPs, or IT staff can also yield the required information. Just reminder to ensure the SMTP server is reachable and not blocked from your network or application to allow smooth email sending.

Now you have some ideas on how to track down your SMTP server’s IP when configuring that new device or addressing those mail delivery problems.

Frequently Asked Questions 

1. Why do I need to find my SMTP IP?
Knowing your SMTP server’s IP address is necessary for properly configuring email clients and devices to route messages through your designated mail servers correctly. It is also useful when troubleshooting mail delivery and connectivity issues between email servers over the internet.

2. Where can I find SMTP details in Office 365 or Exchange Online?
Admins can log into the Office 365 admin center and view details under Admin centers > Exchange. The EAC dashboard shows accepted domains, connectors, mail flow reports and SMTP server details including IPs used for hybrid environments.

3. How can I tell if TCP port 25 connections are being blocked?
Attempt a manual Telnet session to your SMTP server IP on port 25 from your location to check if traffic is allowed or getting blocked by a firewall. The connection should display an SMTP banner if not blocked. Trace routing paths with tracert also.

4. Does SMTP use UDP or TCP ports?
SMTP exclusively uses TCP ports for establishing sessions and transferring email messages reliably over the internet between hops. Main ports are 25 (non-encrypted) and 587 or 465 (submission/implicit SSL).

5. Should I choose an MX record with a lower or higher priority number?
The MX record with the lowest number should be used first for entering into email clients or configuring DNS since that server has the highest sending priority according to DNS. The secondary MX should only be failed over to if issues communicating occur with the primary MX record.

6. What networking terminology relates to mail server administration?Important vocab includes SMTP, POP3, and IMAP for mail protocols, A records, CNAMES, MX records and PTR records in DNS, firewall ports 25/465/587/143/993, routing, TLS/SSL encryption, spam blacklists, DKIM, SPF, DMARC for deliverability, etc.

7. Is Telnet still an effective SMTP diagnostic test?
Yes, Telnet is still useful for testing direct TCP sessions to port 25 to verify basic SMTP banner responses and connectivity despite being an older protocol. Many firewalls still allow port 25 testing via Telnet for troubleshooting purposes.

8. Where are common places to adjust SMTP IP configurations?
Typical apps and services impacted when changing SMTP IPs include desktop mail clients (Outlook, Mac Mail, eM Client), mobile apps, printer email functions, form services, SMTP relay services, analytics, firewall allow lists, etc. Maintain proper PTR records also for reverse resolution of SMTP IPs.

9. Why check multiple MX records on a domain?
Testing connectivity to all published MX records for a domain is useful to identify any mail flow issues related to a specific SMTP server that may be unreliable or unreachable and require troubleshooting against the other MX IPs that are working properly.

10. My emails get sent but end up in spam folders. Why?
If recipients complain of emails in spam folders, common causes include issues with DNS records like SPF/DKIM, needing to request removal from IP blacklists, fixing DMARC policies, or working with Microsoft or ISPs to improve spam filter reputation if legitimate mail is getting flagged by filters unjustly.

11. How can I use A CSV file for bulk domain SMTP lookups?
Nslookup and Telnet allow input redirection from various sources. So SMTP admins can create a CSV file with a list of domains to iteratively pass into nslookup or telnet scripts, parsing the SMTP details returned on each domain into an output report.

12. What are common troubleshooting steps for 403 SMTP errors?
Some things to try are verifying correct username/password configs, testing authentication manually via Telnet, checking for 2FA/app password requirements, looking for SMTP allow listing requirements on ISP IPs, confirming non-standard SMTP ports are permitted properly, etc.

13. Where should firewall ports be opened for external SMTP access?
Inbound internet firewall policies should allow TCP connectivity to destination port 25 for core SMTP, with other common server listening ports including 465 (SMTPS implicit TLS), 587 (SMTP submission), 143 (IMAP) and 993 (IMAP-SSL). 53 (DNS UDP) should have outbound connectivity.

14. Can SMTP and mail data be encrypted without full VPN?
Yes, opportunistic TLS encryption can be implemented for encrypting SMTP sessions and mail data transfer without a client VPN, using SMTP over SSL (SMTPS port 465), TLS on port 587 and SSL-wrapped protocols IMAPS (993) and POP3S (995) for secure mail access.

15. What MX lookup tools besides nslookup provide useful SMTP details?
Admins can use readily available websites like or to perform detailed DNS record lookups and SMTP tests. These tools can save time over running manual queries with nslookup, host, dig and telnet.

16. Why do you need reverse DNS for mail servers?
Proper PTR records mapping SMTP server IPs back to their hostnames are important for deliverability. Mail transfer agents use PTR lookups as a spam filter check to confirm the originating SMTP system is valid with the matching forward/reverse hostname DNS entries for an IP.

17. Can I fake the SMTP banner for Telnet testing?
Using a raw Telnet client you can type anything after connecting to port 25 so spoofing an SMTP response is possible. But an actual 220 banner returned proves two-way communication showing the port is truly open for SMTP flows from that test client’s perspective.

18. How can I find old or alternate SMTP IPs configured?
Check email account settings, login to web hosting control panels to show SMTP configs, review email client connection logs that cache IPs, and trace DNS change history in router configs to identify previously used IPs for comparison if issues started after migrations.

19. Does changing firewalls impact mail delivery?
Switching hardware firewalls in front of SMTP servers can require reconfiguration of SMTP/TCP port allowances in the updated rule sets so that inbound remote mail flow does not get interrupted inadvertently after a firewall device swap.

Leave a Comment