Reference

SMTP reply codes

What each SMTP reply code means, whether it's temporary or permanent, how Emailit handles it, and the replies Emailit's SMTP relay sends to your app.

Updated Oct 1, 2026

Every SMTP command gets a three-digit reply. This page lists the codes you see in two places: replies from recipients’ mail servers when Emailit delivers your mail, and replies from Emailit’s own SMTP relay when your app sends through smtp.emailit.com.

How to read a reply

A reply starts with a three-digit code, usually followed by an enhanced status code and a human-readable message:

Text

550 5.1.1 <ada@example.com>: Recipient address rejected: User unknown

The first digit tells you the outcome:

First digit	Meaning	What the sender should do
`2`	Success. The command was accepted.	Continue.
`3`	Intermediate. The server needs more input, for example the message body after `DATA`.	Send the next part.
`4`	Temporary failure. The same command might work later.	Retry later.
`5`	Permanent failure. Repeating the command won’t help.	Don’t retry without changing something.

The second digit gives the category (0 syntax, 1 information, 2 connection, 5 mail system), and the third digit narrows it down. Servers often use codes loosely, so always read the enhanced code and the message text too.

How Emailit handles replies from recipient servers

When a recipient’s server answers with a failure, Emailit records a delivery with the full reply and decides whether to retry:

Bounced immediately. Replies with code 550, 551, 553 or 554, and replies whose text says the failure is permanent. The email gets the status bounced.
Retried. Everything else, including 421, 450, 451, 452, timeouts and connection errors. The email gets the status attempted and Emailit retries up to 7 times over about 21 hours (after 10, 20, 40, 80, 160, 320 and 640 minutes). If every attempt fails, the email is bounced and the address is suppressed.
Treated as permanent despite a 4xx code. Common “mailbox full”, “over quota”, “user unknown”, “mailbox disabled” and “relay access denied” replies are rewritten to a permanent failure, because retrying them rarely works.
Back-off. A 451 reply pauses delivery from that sending IP to that recipient domain for 5 minutes. A 550 5.7.1 block (for reasons other than content) pauses it for 1 hour. Emails that hit a pause are marked attempted and retried on the normal schedule.

Hard bounces lead to automatic suppression according to your workspace settings. For causes and fixes grouped by problem, see Bounce categories.

Reply code reference

“Emailit” describes what happens when a recipient’s server sends this code during delivery.

2xx and 3xx: success and intermediate

Code	Meaning	Type	Emailit
`220`	Service ready. The server’s greeting, also sent before a `STARTTLS` handshake.	Success	Continues the conversation.
`221`	Closing the connection, usually after `QUIT`.	Success	None.
`235`	Authentication succeeded.	Success	Not used when delivering to recipients; Emailit’s relay sends it to your app after `AUTH`.
`250`	Requested action completed. After `DATA`, the server accepted the message.	Success	Marks the email `delivered`.
`251`	User not local; the server will forward the message.	Success	Treated like `250`.
`252`	The server can’t verify the user but will try to deliver.	Success	Treated like `250`.
`354`	Start sending the message body; end it with a line containing a single dot.	Intermediate	Sends the message.

4xx: temporary failures

Code	Meaning	Type	Emailit
`421`	Service not available, closing the connection. Often too many connections or a temporary reputation block.	Temporary	Retried. Mailbox-full and account-unavailable variants are bounced.
`450`	Mailbox unavailable, for example busy, locked or greylisted.	Temporary	Retried. Quota, unknown-user and disabled-mailbox variants are bounced.
`451`	Local error in processing, often rate limiting or greylisting.	Temporary	Retried, with a 5-minute back-off for that IP and domain. Over-quota and inactive-mailbox variants are bounced.
`452`	Insufficient system storage, or too many recipients in one transaction.	Temporary	Retried. Quota, storage and mailbox-full variants are bounced.
`454`	Temporary authentication or TLS failure.	Temporary	Retried. “Relay access denied” variants are bounced.

5xx: permanent failures

Code	Meaning	Type	Emailit
`500`	Syntax error, command not recognized.	Permanent	Retried, then bounced after the last attempt.
`501`	Syntax error in parameters or arguments, such as a malformed address.	Permanent	Retried, then bounced after the last attempt.
`502`	Command not implemented.	Permanent	Retried, then bounced after the last attempt.
`503`	Bad sequence of commands.	Permanent	Retried, then bounced after the last attempt.
`504`	Command parameter not implemented.	Permanent	Retried, then bounced after the last attempt.
`521`	The domain doesn’t accept mail (RFC 7504).	Permanent	Retried, then bounced after the last attempt.
`530`	Authentication required, or the server requires TLS first.	Permanent	Retried, then bounced after the last attempt.
`534`	Authentication mechanism is too weak.	Permanent	Retried, then bounced after the last attempt.
`535`	Authentication credentials invalid.	Permanent	Retried, then bounced after the last attempt.
`538`	Encryption required for the requested authentication mechanism.	Permanent	Retried, then bounced after the last attempt.
`550`	Mailbox unavailable: the address doesn’t exist, or the server refused the message for policy or spam reasons.	Permanent	Bounced. `550 5.7.1` blocks also trigger a 1-hour back-off for that IP and domain.
`551`	User not local; the server won’t forward.	Permanent	Bounced.
`552`	Mailbox full or message exceeds the server’s size limit.	Permanent	Retried, then bounced after the last attempt. Known over-quota replies are bounced immediately.
`553`	Mailbox name not allowed, for example an invalid address.	Permanent	Bounced.
`554`	Transaction failed, often a policy, spam or reputation rejection. Also sent as a greeting when a server refuses the connection.	Permanent	Bounced.
`555`	`MAIL FROM` or `RCPT TO` parameters not recognized.	Permanent	Retried, then bounced after the last attempt.
`556`	The domain doesn’t accept mail (RFC 7504).	Permanent	Retried, then bounced after the last attempt.

Replies from Emailit’s SMTP relay

When your app sends through smtp.emailit.com, these are the replies Emailit can return. Replies to MAIL and DATA commands on an authenticated connection are also listed in Email APILogs with their status code.

Reply	Command	Cause	What to do
`235 Authentication successful`	`AUTH`	The API key was accepted.	Continue.
`535 Authentication failed`	`AUTH`	The password isn’t a valid API key for any workspace.	Use a current API key as the password. The username can be `emailit`.
`454 Temporary authentication failure`	`AUTH`	Emailit couldn’t check the key because of an internal error.	Retry after a short delay.
`504 Error: Unrecognized authentication type`	`AUTH`	Your client used a method other than `PLAIN` or `LOGIN`, such as `CRAM-MD5`.	Set the client to `PLAIN` or `LOGIN`.
`452 4.4.5 Messages per second limit exceeded (n/limit)`	`MAIL FROM`	The workspace hit its per-second sending limit. The limit is shared by the API and SMTP.	Slow down and retry. Most mail libraries retry `4xx` replies automatically.
`452 4.5.3 Daily message limit exceeded (n/limit)`	`MAIL FROM`	The workspace hit its daily sending limit, which resets at 00:00 UTC.	Wait for the reset or request a higher limit from the dashboard.
`451 Temporary local error in processing`	`MAIL FROM`, `RCPT TO`, `DATA`	A temporary problem on Emailit’s side.	Retry later.
`530 Authentication required`	`RCPT TO`	The client didn’t log in before sending.	Enable SMTP authentication in your client.
`501 Invalid RCPT TO format`	`RCPT TO`	The recipient address is malformed.	Fix the address.
`550 Unverified workspaces can only send to workspace members' account emails.`	`RCPT TO`	The workspace is in sandbox mode and the recipient isn’t a member.	Send to a member’s account email, or request production access.
`535 Mail server has been suspended`	`RCPT TO`	The workspace is suspended.	Check your sending health and contact support.
`530 From/Sender domain is not verified for this workspace. From: ...`	`DATA`	The `From` header isn’t on a verified sending domain of the workspace. Subdomains must be verified separately.	Verify the domain or change the `From` address.
`530 API key is restricted to sending domain: acme.com. ...`	`DATA`	The API key is restricted to one domain and the `From` address uses another.	Use the allowed domain or a different key.
`550 Sending from this domain is paused`	`DATA`	The domain was paused, usually because of a high bounce rate.	See Sending health.
`552 Message too large (maximum size 40MB)`	`DATA`	The message, including encoded attachments, is over 40 MB.	Send smaller attachments or link to files instead.
`550 Loop detected`	`DATA`	The message already passed through Emailit’s relay more than 4 times.	Check forwarding rules that send mail back to Emailit.
`550 Message processing failed`	`DATA`	Emailit couldn’t store the message.	Retry. If it keeps failing, contact support with the time of the attempt.
`250 2.0.0 OK: queued as em_...`	`DATA`	Emailit accepted the message. Each recipient gets its own email ID, listed comma-separated when they fit in the reply.	Store the ID to look the email up in the dashboard or API.

The relay also sends standard protocol replies, such as 220 when you connect, 503 Error: need MAIL command when commands arrive out of order, and 421 Timeout - closing connection when a connection sits idle.