Best SMTP test tools
Five SMTP test tools ranked on what they show about a mail server, whether they work where port 25 is blocked, and whether they avoid sending mail.
How this was compared
Disclosure: ZapBounce is our product. Figures were read from each vendor's own pages in September 2026, and the criteria below were applied to every entry, ours included.
The short version
- Where the test runs from: a server with outbound port 25 open, or your own machine, which most home networks and many cloud hosts block on port 25.
- What it shows: the MX hosts, the 220 banner, the EHLO extensions, STARTTLS support and the response time.
- Whether it stays clear of sending a message, so testing a server never lands mail in anyone's inbox.
- Cost and setup: free in a browser, or an install and a command line. Each tool's own documentation was read on 25 September 2026.
- Disclosure: ZapBounce is our product, and the same criteria were applied to it.
At a glance
| 10,000 addresses | Credit expiry | Bills unknowns | |
|---|---|---|---|
| ZapBounce SMTP test | $25 | Credits never expire | No |
| MxToolbox SMTP Diagnostics | Not a verification vendor | ||
| Swaks | Not a verification vendor | ||
| OpenSSL s_client | Not a verification vendor | ||
| Telnet | Not a verification vendor |
Vendor figures read from their own pricing pages in September 2026. Confirm before purchase, because prices move.
The ranking
Best first, with the situation each one fits.
ZapBounce SMTP test
Disclosure: ZapBounce is our product. Type a domain and the test looks up its MX hosts, connects on port 25 from our servers, reads the 220 banner and the EHLO reply, checks STARTTLS and times the response. It then says QUIT, so no sender, recipient or message is ever given.
- Best for:
- Anyone checking why mail to a domain fails, from any network, with no install. Start at the SMTP test, confirm encryption with the email TLS checker, and read why port 25 is blocked on home connections.
MxToolbox SMTP Diagnostics
MxToolbox's SMTP diagnostic runs a simple open relay test, checks that the server has a reverse DNS (PTR) record and measures its response times, according to mxtoolbox.com on 25 September 2026.
- Best for:
- Mail admins checking their own server for an open relay and a missing PTR record.
Swaks
Swaks is a scriptable SMTP test tool, licensed under the GNU GPLv2, that supports TLS, authentication and pipelining and speaks SMTP, ESMTP and LMTP (jetmore.org, read 25 September 2026).
- Best for:
- Engineers scripting full SMTP transactions against their own relay, including authenticated sends on port 587.
- Less suited to:
- Home networks, since it runs from your machine and most residential providers block outbound port 25.
OpenSSL s_client
The OpenSSL command line opens a connection with -starttls smtp and prints the server's certificate chain and the negotiated TLS version.
- Best for:
- Checking a mail server's certificate and TLS setup by hand, on a machine that already has OpenSSL.
- Less suited to:
- Home networks, since it runs from your machine and most residential providers block outbound port 25.
Telnet
Telnet opens a plain-text connection to port 25, so you type EHLO, MAIL FROM and RCPT TO yourself and read each reply code.
- Best for:
- Learning the SMTP conversation one command at a time.
- Less suited to:
- Testing STARTTLS, since telnet has no way to negotiate TLS once the server offers it.
Why ZapBounce ranks first
How to choose
- Your network: Swaks, OpenSSL and telnet run from your own machine, where port 25 is often blocked. ZapBounce's SMTP test runs from our servers, so it reaches the mail server from any network.
- Encryption: telnet has no TLS, and OpenSSL shows the certificate once you pass -starttls smtp. ZapBounce reports STARTTLS support in the same result as the banner and the MX hosts.
- Relay and PTR checks: MxToolbox's diagnostic tests for an open relay and a PTR record (read 25 September 2026). ZapBounce's test reads the banner and EHLO reply and closes with QUIT, so it never asks the server to carry a message.
- Mailbox checks: a server test says the server answers. ZapBounce's email verifier goes one step further and asks about a single recipient, returning valid, invalid, catch_all or unknown.
Questions people ask
How do I test an SMTP server online?
Enter the domain in an online SMTP test that runs from a server with port 25 open. A working server answers with a 220 banner, replies to EHLO with 250 and a list of extensions, and offers STARTTLS if it supports encryption.
Why does telnet to port 25 time out from my computer?
Most residential internet providers and many cloud hosts block outbound port 25 to stop spam from infected machines. The mail server is probably fine; your network is closed. A test that runs from a server with port 25 open gets past it.
Does an SMTP test send an email?
A server test does not. It opens the session, reads the banner and the EHLO reply, checks STARTTLS and says QUIT, so no sender, recipient or message is ever given.
What is the difference between an SMTP test and email verification?
An SMTP test checks that a domain's mail server answers and how it is set up. Email verification goes one step further in the same conversation and asks the server about one recipient with RCPT TO, then closes before any message is sent.
Which port should I test, 25 or 587?
Port 25 is where a domain's inbound server receives mail from other servers, so that is the one a delivery problem points to. Port 587 is for your own outgoing relay and needs your username and password, which you should never type into a web tool.
Run the same sample through two of them
100 free checks a month, no card. Compare how many addresses each one declined to judge, which matters more than either accuracy claim.