Result
How the check works
First we use the domain’s MX records to find the server that receives its email and check whether it is reachable on port 25. We then start the usual SMTP dialogue with that server: greeting (EHLO), then – if offered – switching to encryption (STARTTLS) and greeting again. If you only enter a domain, we then end the session with QUIT.
With a complete email address the mailbox check follows: we announce an empty sender (MAIL FROM:<>) and name the address as recipient (RCPT TO). The server’s reply shows whether it accepts mail for that mailbox. We then abort with RSET and QUIT. We never send a message (DATA) – nobody will see anything.
You can see the complete dialogue in the result. We also show the server’s capabilities as well as the TLS version, cipher and the certificate with issuer, validity and host names.
Reading the result correctly
If a mail server does not offer STARTTLS, email to this domain is transmitted unencrypted. A certificate that does not match the mail server’s name or has expired is common with email, but weakens protection – especially if the domain wants to use MTA-STS or DANE.
Mailbox check: 250 means “accepted”, 550 with 5.1.1 “unknown mailbox”, 552, 452 or 5.2.2 “mailbox full”. Other 4xx replies are temporary, often greylisting – check again later. Important: checking a complete email address is only an indication, not proof that the address can actually be reached. Mailboxes are often only checked further downstream in the responsible mail server system – after the address was accepted in the dialogue. Catch-all domains also accept any address, and some servers only reject later, for example when the mailbox is full.
If a server already refuses our connection or the query (e.g. 5.7.1), this is often the provider’s policy towards test servers and says nothing about whether normal email is delivered.
Frequently asked questions
Is an email sent during the check?
No. Even with the mailbox check we abort after naming the recipient (RCPT TO) and never send a message (DATA). Nobody will see anything.
Can I check whether a mailbox is full or exists?
Yes, enter the complete email address. We show whether the mail server accepts the mailbox, reports it as full or unknown, and the server’s exact reply. To prevent misuse: one address per query, at most 5 mailbox checks per day.
Why is “accepted” no guarantee?
The mailbox check is only an indication, not proof: many mail server systems only check the mailbox further downstream, after accepting the address in the dialogue. Some domains accept any address (catch-all) and sort it out later. Other servers only check storage or filters when the message arrives. Only a real email with a reply gives a definite answer.
Why does it say “no connection to port 25”?
Either the mail server is currently unreachable or it does not let our test server in. Many networks also block outgoing connections on port 25 to prevent spam.
Domains and DNS in good hands
1blu business GmbH looks after domains, DNS and mail servers for companies, agencies and resellers – with dedicated contacts instead of a call centre.