SMTP Test
This free SMTP test connects to a domain's mail servers on port 25 and checks that they answer, offer STARTTLS encryption with a valid certificate and have reverse DNS set. It never sends an email.
- MX hosts
- Greeting
- STARTTLS
- TLS certificate
Free, no signup. Try it with github.com.
What the SMTP test does
We look up the domain's MX records and connect to up to three of its mail servers on port 25, from our server in Germany. Each one should greet us, list what it supports and agree to switch to encryption (STARTTLS).
Then we say goodbye. We never give a sender or a recipient, so no email is sent and nobody's address is checked.
Why STARTTLS and reverse DNS matter
Without STARTTLS, mail to your domain crosses the internet as plain text. Most mail servers offer it today, and policies like MTA-STS require it to work with a valid certificate.
Reverse DNS (a PTR record) ties the server's IP address back to a name. Big mailbox providers distrust servers without one, so it matters for the mail you send as much as for the mail you receive.
If a mail server doesn't answer
Check that the MX record points at the right host name and that port 25 is open in the server's firewall. Home and office connections often block port 25, which is why mail servers live in data centres.
If only one of several servers fails, mail still arrives through the others, but fix it anyway: senders may try the broken one first, and delivery slows down.
Do it on your hosting
Step-by-step guides from our knowledge base.
- How to Track Email Delivery in cPanelKnowledge base
- Why Are My Emails Being Blocked and How to Improve DeliverabilityKnowledge base
Frequently asked questions
Does this test send an email?
No. We connect, say hello (EHLO), try STARTTLS and disconnect. We never name a sender or a recipient, so nothing is delivered and no address is checked.
Why only port 25?
Port 25 is where mail servers receive email from other servers. Ports 587 and 465 are for your own email app sending mail, and they need a login.
What does it mean when STARTTLS fails?
The server offered encryption but the secure handshake didn't work, often because of a broken or expired certificate. Depending on their settings, senders then deliver without encryption or not at all.
How often can I run the test?
Each visitor can run 2 tests a minute and 10 an hour, and each domain can be tested 10 times an hour. When the mail servers answered, we keep the result for 10 minutes, and repeat views of it don't count.
The servers answer, but email still bounces. Why?
A server can be reachable and still refuse mail, for example because of SPF or DMARC, or a full mailbox. Run our email deliverability check to see the DNS side.
Dig deeper
MX Lookup
See which servers receive email for a domain, in priority order, and which provider runs them.
Look up MXSPF Record Checker
Validates your SPF record and counts lookups against the hard limit of 10, before one more include breaks it.
Check SPFEmail Deliverability Checker
One scan, eight checks: MX, SPF, DKIM, DMARC, MTA-STS, TLS-RPT, BIMI and blocklists. Scored out of 100, with a fix for every gap.
Score your domain