SMTP Connectivity Testing Guide

Modified on Fri, 25 Jul at 7:00 PM

This guide provides command line instructions to test SMTP connectivity to outbound.mailhop.org and troubleshoot communication issues.

What a Successful Connection Looks Like:

When testing connectivity, you should see an SMTP banner response like this:

Command:  nc outbound.mailhop.org 25

Response:  220 outbound4.ore.mailhop.org ESMTP

This confirms the port is open and the SMTP service is responding properly.

Available SMTP Ports:

Port 25: Standard SMTP (may be blocked by ISPs)
Port 587: Submission port with STARTTLS
Port 2525: Alternative submission port
Port 465: SMTP over SSL/TLS

Windows Commands

Test port connectivity using Telnet:

telnet outbound.mailhop.org 25

telnet outbound.mailhop.org 587

telnet outbound.mailhop.org 2525

telnet outbound.mailhop.org 465

Test port connectivity using PowerShell:

Use this method if telnet is not available on the system:

Test-NetConnection -ComputerName outbound.mailhop.org -Port 25

Test-NetConnection -ComputerName outbound.mailhop.org -Port 587

Test-NetConnection -ComputerName outbound.mailhop.org -Port 2525

Test-NetConnection -ComputerName outbound.mailhop.org -Port 465

Mac/Linux Commands

Test port connectivity using Telnet:

telnet outbound.mailhop.org 25

telnet outbound.mailhop.org 587

telnet outbound.mailhop.org 2525

telnet outbound.mailhop.org 465

Test port connectivity using Netcat (nc):

nc -zv outbound.mailhop.org 25

nc -zv outbound.mailhop.org 587

nc -zv outbound.mailhop.org 2525

nc -zv outbound.mailhop.org 465

SWAKS (Swiss Army Knife for SMTP)

SWAKS is a comprehensive SMTP testing tool that provides detailed protocol-level testing capabilities.

Installation:

Windows: Download from http://www.jetmore.org/john/code/swaks/
Mac: brew install swaks or download from the website
Linux: apt-get install swaks or yum install swaks

Basic SWAKS Test Examples:

Test basic connectivity and SMTP capability:

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 25

Test with authentication (if required):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 587 --auth --auth-user username --auth-password password

Test TLS/SSL connectivity:

For STARTTLS (ports 25, 587, 2525):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 587 --tls

For SSL/TLS (port 465):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 465 --tlsc

Test all ports with verbose output:

Port 25 (standard SMTP):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 25 -v

Port 587 (submission with STARTTLS):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 587 --tls -v

Port 2525 (alternative submission with STARTTLS):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 2525 --tls -v

Port 465 (SMTP over SSL/TLS):

swaks --to test@example.com --from sender@example.com --server outbound.mailhop.org --port 465 --tlsc -v

Expected Results

Successful Connection:

You should see the SMTP banner, typically something like:

220 outbound.mailhop.org ESMTP...

Common Issues:

Connection refused: Port is blocked or service isn't running on that port
Timeout: Network connectivity issue or firewall blocking the connection
DNS resolution failure: Unable to resolve outbound.mailhop.org

Troubleshooting Tips

Start with basic telnet/nc tests to verify port accessibility
Use SWAKS for comprehensive SMTP protocol testing
Test from different network locations if possible
Check firewall settings on both client and server sides
Verify DNS resolution: nslookup outbound.mailhop.org
Many ISPs block port 25 - try alternative ports 587, 2525, or 465

Note: The telnet and nc tests confirm basic port accessibility, while SWAKS provides comprehensive SMTP protocol testing including authentication and encryption capabilities.