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SMTP Test
Check SMTP readiness signals (MX, SPF, DMARC) and learn how to test SMTP connectivity on ports 25/465/587 from your server.
Best default
Port 587
STARTTLS submission is recommended.
Note
Browser limitations
Connectivity needs a server-side test.
Waiting for input
Enter a domain, then press Check
How to Use
Use SMTP Test in 4 Steps
01
Enter domain
Type the domain you’re sending email from.
02
Check signals
We validate DNS signals: MX, SPF, and DMARC presence.
03
Pick a port
Use 587 (recommended), 465 (TLS), avoid 25 when blocked.
04
Test from server
Run the provided openssl commands from your sending environment.
What is SMTP Test?
SMTP Test helps teams diagnose email sending issues and common ports. This page is built for production troubleshooting, migration validation, and repeatable change verification workflows.
When to Use SMTP Test
- During pre-change validation to baseline expected behavior
- During active incidents to isolate likely root-cause signals
- During post-change verification to confirm remediation success
- During audit/compliance reviews where reproducible evidence is required
How to Interpret Results
Run this tool with clean input, compare output with known-good state, and cross-check adjacent dependencies. Avoid single-signal conclusions when resolver variance, provider policy, or caching windows may influence observations.
Operational Recommendations
- Validate input formatting and scope before execution.
- Re-run checks from shared URLs for team handoff consistency.
- Confirm final conclusions with authoritative provider logs.
- Document changes with before/after evidence for incident timelines.
SMTP Test in EMAIL Workflows
Use this route as a first-pass check in your email diagnostics stack. Then continue with related tools to validate surrounding layers and prevent false positives from partial visibility.
Frequently Asked Questions
Can a browser test SMTP connectivity?
Not reliably. Browsers cannot open raw TCP connections to SMTP ports, so connectivity testing needs a server-side tool.
Which SMTP port should I use?
Port 587 (STARTTLS submission) is recommended. Port 465 is implicit TLS and still common. Port 25 is often blocked.
Why is port 25 blocked?
Many ISPs and cloud providers block port 25 to reduce spam. Use port 587 with authentication instead.
Do SMTP issues relate to SPF/DKIM/DMARC?
They are different layers. SMTP is connectivity/authentication. SPF/DKIM/DMARC affect acceptance and inbox placement.
What should I check first if sending fails?
Confirm SMTP host and credentials, try port 587, and verify firewalls. Then validate SPF/DKIM/DMARC for deliverability.
Ports
587 submission (STARTTLS)
465 implicit TLS
25 relay (often blocked)