SMTP transcript explainer: read a mail server's replies in plain English
In short: Paste the lines of a hand-run SMTP session. Each server reply gets its RFC 5321 meaning, each RCPT TO is marked accepted, temporary or rejected, and a made-up address that was accepted points to a catch-all server. Limit: It reads text you paste; it does not open a connection or test anything itself.
Paste an SMTP session from openssl, nc or telnet and the tool pairs every command with the server's reply and gives the meaning of each three-digit code from RFC 5321. It marks each RCPT TO as accepted, temporary or rejected. If you name an address that cannot exist and the server said 250, that server accepts everything.
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If the server accepted it, the server accepts any address (catch-all).
Sample result
Two addresses at example.net were accepted, including one made up.
- Catch-all sign
- Yes
- Replies read
- 6
- Addresses tried
- 2
What this means for you
A made-up address that gets a 250 is the sign of a catch-all server.
- RFC 5321 wording
- RCPT TO outcomes
- Made-up address test
- Copy and download
- SMTP
- Catch-all
- Terminal
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
A hand-run session tests a few addresses. A list needs a check for each one.
- Collect the addresses you want to test, one per line or in a CSV.
- Upload or paste it at /verify. Every address gets a live mailbox check, and the first 100 email checks are free with a Google sign-in, no card.
How do I tell from an SMTP transcript if a server is catch-all?
- Run an SMTP session by hand with at least one real address and one made-up address, for example with the SMTP test command generator.
- Paste the whole capture, your lines and the server's replies.
- Type the made-up address in the optional box.
- Read the verdict and the table of replies, then copy or download them.
Example
- You enter
- Example session with mx.example.net: a 220 greeting, EHLO, MAIL FROM, two RCPT TO lines (ava@example.net and the made-up zq7x3kd9v1@example.net, both answered 250) and QUIT.
- You get
- Six replies read, 2 addresses tried and 2 accepted. The made-up address zq7x3kd9v1@example.net was accepted with 250, which is the sign of a catch-all server.
- Limits
- Runs in your browser and reads the text you paste; it does not connect to any server.
What does each reply code mean?
The first digit is the outcome: 2 means the command worked, 3 means the server wants more, 4 means a temporary failure and 5 means a permanent one. The tool shows the wording RFC 5321 gives for each code, such as 250 for requested mail action okay and 550 for mailbox unavailable, and falls back to the first digit for codes it has no wording for.
Many servers add an enhanced code such as 5.1.1 after the three digits. The tool shows it beside the reply. To look one up, use the bounce message analyzer.
How does it spot a catch-all server?
A catch-all server says yes at RCPT TO to any address on its domain. So if you tried an address nobody could own, such as a long random string, and the reply was 250, the domain accepts everything and a 250 for a real address proves nothing. If the made-up address was rejected, the server tells addresses apart.
To test many domains for this at once, use the bulk catch-all checker.
What does a 250 not tell you?
Accepting an address at RCPT TO is not the same as delivering to an inbox. Some servers accept everything and bounce later, and some answer 252 when they cannot confirm a user. Read SMTP verification false positives before you rely on one probe.
Who uses this tool, and for what?
- Cold email senders
- Explain what a server said before you decide whether an address is worth sending to.
- Deliverability engineers
- Turn a pasted session into a plain-English list of replies for a ticket or a client.
- Sales ops
- Check whether a key account's domain accepts every address before you trust its mailbox checks.
What can’t this tool tell you?
- It reads text you paste; it does not open a connection or test anything itself.
- One probe on one connection is a signal, not proof, because servers can answer differently over time.
- Reply wording is the standard text from RFC 5321; a server's own words are shown beside it as written.
- Enhanced status codes are shown but not decoded here.
Common questions
- What formats can I paste?
- Output from openssl s_client, nc and telnet works, with or without prefixes such as S:, C:, > and <. Lines that are neither a reply nor an SMTP command are ignored.
- Does it connect to the mail server?
- No. It only reads the text you paste, in your browser.
- What if the replies are 450 or 451?
- Those are temporary. The tool gives no verdict on those addresses and suggests trying again later.
- Does a 550 always mean the mailbox is missing?
- No. RFC 5321 also uses 550 for policy rejections. A 5.7.x code next to it usually points to a block, not a missing mailbox.
- Can it find a catch-all server without my made-up address?
- Only when one address is rejected and another accepted, which shows the server tells them apart. Otherwise it needs the made-up address to decide.
- Is my transcript uploaded?
- No. It stays in your browser.
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? Starter at $9/mo covers 2,500 emails a month, cancel anytime
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.