SMTP Error 542

Is "SMTP Error 542" a real SMTP code?

No. There is no reply code 542 in the SMTP standard. RFC 5321 §4.2.3 lists every reply code the protocol defines, and 542 is not among them. The structure of the code is also a giveaway: in RFC 5321 §4.2.1–4.2.2 the first digit is the outcome (2yz success, 4yz transient failure, 5yz permanent failure) and the second digit is the subject group — x0z syntax, x1z information, x2z connections, x5z mail system. The second digit 4 (as in 542) is unassigned in the standard, so a spec-compliant server does not emit it.

"542" appears on a number of email-tooling blogs labeled "bad connection," but none cite an RFC or a mainline provider (SendGrid, Amazon SES, Mailgun, Postmark) that actually returns it. Treat the label as noise. A few servers or anti-spam filters may put the digits 542 inside a custom rejection string (e.g. 542 Rejected), but that is a vendor-specific message, not a standard reply code — and the fix depends entirely on the human-readable text that follows it.

How do I fix the rejection behind "SMTP Error 542"?

Stop searching for "542" and find the real code your server logged.

1. Read the full server response. Look in your SMTP transcript or provider logs for the complete line, e.g. 550 5.7.1 ... or 421 4.7.0 .... The first digit tells you whether to retry: 4yz is transient (retry later), 5yz is permanent (fix and resend). The trailing dotted number is the RFC 3463 enhanced status code (e.g. 5.7.1 = delivery-policy/authorization failure).
2. Match the code to its real cause. Common permanent rejections that get mislabeled as a generic "bad connection":
3. 550 5.7.1 / 554 5.7.1 — policy or reputation block. Authenticate your mail with SPF, DKIM, and DMARC, and check whether your sending IP/domain is on a blocklist.
4. 530 5.7.0 (RFC 4954 §6) — authentication required. Supply valid SMTP credentials.
5. TLS negotiation failures — connect on port 587 (STARTTLS) or 465 (implicit TLS); both are valid per RFC 8314. Neither port is deprecated.
6. For Gmail / Google Workspace SMTP, authenticate with an App Password (requires 2-Step Verification) or OAuth2. Google removed "Less Secure Apps" access in 2022 — do not rely on it.
7. If it's a vendor 542 Rejected string, act on the words after the number and consult that product's docs; it is a local policy decision, not a protocol error.