Every SMTP error starts with a three-digit code. Codes starting with 4 are temporary: the server is saying "not now, try again later", and mail systems usually retry. Codes starting with 5 are permanent: the request was refused and repeating it won't help until something changes. Read the text after the code. It usually says whether the problem is your login, the recipient address, your sending reputation, or a policy such as missing authentication.
What's the problem
Your app or email client shows an error with a code, such as "550 Requested action not taken: mailbox unavailable", or a connection timeout when sending.
Why it happens
- Login problems: wrong username, password or API key, or the account requires an app password.
- Connection problems: the wrong host or port, or your hosting provider blocks outgoing SMTP ports.
- The recipient doesn't exist or their mailbox is full.
- Policy rejections: missing SPF or DKIM, a poor sending reputation, or content flagged as spam.
- Rate limits: too many messages too fast, or a daily limit reached.
How to fix it
- Read the full error: the code and the text.
- For authentication errors, check the SMTP username and password or API key, and whether the service requires a specific key type.
- For timeouts, check the host and port your email service documents, and whether your hosting blocks that port.
- For 5xx rejections about the recipient, check the address.
- For policy rejections, set up SPF and DKIM for your domain. See emails not sending.
- For 4xx codes, let the mail system retry, and slow down if you're hitting rate limits.
What Preventionlabs checks
One click runs the full report on your domain. For this problem it shows your email authentication: SPF (and its 10-lookup limit), DMARC and DKIM; whether mail to you must arrive encrypted (MTA-STS) and whether you get reports when it does not; your DNS: nameservers, addresses, DNSSEC, and records pointing at services that no longer exist. It can't see your sending provider's delivery logs, so check bounces there.
Run your free reportYour first report is free, no card needed. Then A$50 a month, up to 10 domains. Read-only: it never changes anything.
Sources
- IETF: Simple Mail Transfer Protocolstandard (RFC 5321)
The command was not accepted, and the requested action did not occur. However, the error condition is temporary, and the action may be requested again.
- IETF: Simple Mail Transfer Protocolstandard (RFC 5321)
The command was not accepted and the requested action did not occur. The SMTP client SHOULD NOT repeat the exact request (in the same sequence).
- Twilio SendGrid: SMTP Errors and Troubleshootingofficial docs
550 Requested action not taken: mailbox unavailable