Test whether a mail server answers, what its banner and EHLO reply offer, and whether it accepts mail for an address, all without sending a message. Note: this checker runs from shared hosting that blocks outbound port 25, so port 25 tests time out from here, while ports 587 and 465 test normally.
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SMTP, short for Simple Mail Transfer Protocol, is the protocol mail servers use to pass email to each other. When you send a message, your provider hands it to a mail server, and that server uses SMTP to deliver it to the server named in the MX records of the recipient domain. This checker speaks the first part of that conversation with a mail server and reports exactly what the server says back.
Port 25 is the server to server port. Receiving servers listen on port 25 for mail coming from other servers, and it is the port this checker uses in domain mode against the MX host of a domain.
Port 587 is the submission port. Mail clients and apps use it to hand outgoing mail to their provider, normally after a STARTTLS upgrade and with a login.
Port 465 is the implicit TLS port. The connection is encrypted from the first byte, with no plain text greeting and no STARTTLS step.
The banner is the first line a server sends when you connect. It usually starts with code 220 and names the server software or the provider. After the banner, this checker sends EHLO and the server answers with its capability list. STARTTLS means the server can upgrade the connection to encrypted TLS. AUTH lists the login methods it offers. SIZE states the largest message size it accepts. Together, the banner and the EHLO list tell you whether the server is alive, how it expects to be talked to, and whether it can take mail securely.
This checker runs on shared hosting, and that hosting blocks outbound connections on port 25. Tests on port 25, including domain mode tests against MX servers, time out from here even when the server being tested is healthy. That is a limit of this hosting, not a result about your server. Ports 587 and 465 are not blocked, and tests on those ports run normally.
Sending a message takes more steps than this checker performs. After EHLO, a real send continues with MAIL FROM, then RCPT TO, then DATA, and only the DATA step carries a message. This checker never sends DATA. When you give a recipient address, it asks the server one question, in effect: would you accept mail for this address. The server answers with a code, the checker records the code, sends QUIT, and disconnects. No message is written, sent, or delivered.
A timeout on port 25 most often means a network on the path blocks port 25, which many hosts and internet providers do to fight spam. It does not prove the target server is down.
A 4xx answer to the RCPT probe, such as 450 or 451, is a temporary refusal. Greylisting works this way on purpose: a server that does not know the sender asks it to try again later. A 4xx answer is not proof that an address does not exist.
A 5xx answer, such as 550, is a permanent refusal, for example because the mailbox does not exist or the server will not relay for you.
Warning signs of an open relay: a server that answers 250 to a RCPT probe for an outside domain it should not serve, from a sender it has no reason to trust, may accept and forward mail for anyone. Open relays get abused for spam and get listed on blacklists fast. If your server behaves this way, close it to relaying for strangers.
Only test servers you own or have permission to test. Probing a server you have no rights to can break its rules and, in some places, the law.
To find the MX server of a domain first, use the MX Record Checker. To read the path a received message took, use the Email Header Analyzer. For a full picture of a domain, run the Domain Health Checker.
It connects to a mail server, reads its banner, sends EHLO, lists its capabilities, checks whether the connection is encrypted with TLS, and, if you give a recipient address, asks whether the server would accept mail for that address. In domain mode it tests the highest priority MX server of the domain on port 25.
No. The checker never sends the DATA command, so no message is written or delivered. It ends every test with QUIT.
Port 25 is blocked by many networks to fight spam, and the shared hosting this checker runs on blocks outbound port 25 as well. A timeout on port 25 from here says the path is blocked. It does not say the server is down. Test the same server on port 587 or 465 in direct mode.
STARTTLS is a command that upgrades a plain connection to an encrypted TLS connection after the first greeting. On port 587 the conversation starts in plain text and upgrades with STARTTLS. On port 465 the connection is encrypted from the start, so there is no STARTTLS step.
It means the server accepted that recipient at the RCPT step. The address passed this check and the server would take a message for it, if one were sent. None is sent by this checker.
A 550 is a permanent refusal. Common reasons are that the mailbox does not exist, the domain is not served here, or the server refuses to relay for you.
Yes. You can test the Gmail MX servers like any public mail server, for example in domain mode with gmail.com, or in direct mode against smtp.gmail.com on port 587 or 465. Port 25 tests time out from this hosting, as explained above.
Only test servers you own or have permission to test. Public MX servers are built to answer EHLO from strangers, but repeated probing of a server you do not own can still break its terms. When in doubt, do not test it.
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