SMTP error: data not accepted

What is "SMTP Error: data not accepted"?

This is not a standard SMTP reply code — it is a generic error string produced by the PHPMailer library (and tools built on it, such as many WordPress SMTP plugins, WHMCS, and Mailster). PHPMailer emits it whenever the SMTP server returns a non-250 reply after the message body has been sent — i.e., after the terminating <CRLF>.<CRLF> of the DATA command (RFC 5321 §4.1.1.4).

In other words, the connection, authentication, MAIL FROM, and RCPT TO steps all succeeded — the server accepted the envelope and then rejected the message content. The actual reason is in the server's reply code, which PHPMailer hides behind this friendly string.

What causes "SMTP error: data not accepted"?

Per RFC 5321 §4.2.3, which enumerates reply codes in numeric order, the server's reply at end-of-DATA tells you the real cause. The most common ones:

• 550 / 554 — content or policy rejection (most common). The receiving server's spam/policy filter rejected the message. A real-world PHPMailer report showed 550 This message was classified as SPAM and may not be delivered (PHPMailer #1370). 554 is a generic "transaction failed," often an unverified/unauthorized sender.
• 552 — message size exceeds the server limit. Large HTML or attachments push past the server's SIZE limit.
• 451 / 421 — temporary failure or rate/quota throttling. Sending too fast or a transient server error; these are retryable.
• From / sender alignment. Some relays reject the body if the From: header (or Reply-To) doesn't match the authenticated SMTP user or an SPF/DKIM/DMARC-aligned domain. This is one specific case of a 550/554 policy rejection — not the universal cause.

How do I fix "SMTP error: data not accepted"?

Step 1 — Read the real reply code. This is the single most important step. In PHPMailer, turn on debug output before sending:

$mail->SMTPDebug = SMTP::DEBUG_SERVER; // verbose client/server dialog

Look in the dialog for the line right after the message body, e.g. SERVER -> CLIENT: 550 .... Everything below keys off that code. You can also inspect $mail->ErrorInfo.

Step 2 — Fix by code:

• 550 / 554 spam or policy: Authenticate your domain. Publish SPF (RFC 7208), DKIM (RFC 6376), and DMARC (RFC 7489) records for the sending domain, and make sure your From: address uses that domain. Review content for spam triggers and warm up new IPs. If a specific recipient server blocks you, ask their admin to allowlist you.
• From/username mismatch: Set the From: (and Reply-To) to an address the SMTP account is authorized to send as — typically the authenticated user or a verified sender on your provider. In PHPMailer use $mail->setFrom('you@yourdomain.com', 'Name') with an actual address, not just a display name.
• 552 size: Reduce attachment/body size, or send large files as links. Check your provider's max message size.
• 451 / 421 throttling: Back off and retry with exponential delay; slow your send rate to stay under the provider's per-minute/hour quota.

Step 3 — Verify provider auth (if relevant). For personal Gmail, use an App Password (requires 2-Step Verification) or OAuth2 — "Less Secure Apps" was removed in 2022 and is no longer an option. For Google Workspace accounts, Basic Authentication (username/password, including app passwords in many org configurations) for SMTP was phased out starting March 2025 in favor of OAuth2. If you're sending transactional mail via PHPMailer/WHMCS/WordPress from a Workspace account, either implement OAuth2 or use Google Workspace's dedicated SMTP relay service (smtp-relay.gmail.com), which authenticates by sender IP/TLS rather than a user password — check with your Workspace admin about current policy before assuming an app password will work. Ports 587 (STARTTLS) and 465 (implicit TLS, RFC 8314) are both valid.

Why this differs from a connection or auth error

"Data not accepted" specifically means the handshake and recipient acceptance worked and the content was refused. If authentication itself failed you'd see 535; if a recipient was unknown you'd see 550 5.1.1 at RCPT TO (before DATA). Use the debug dialog to confirm which stage failed.