Error Codes, Delivery Logs & Diagnostic Reference

This technical reference provides an exhaustive mapping of internal delivery status codes, standard SMTP response codes, retry and failure recording mechanics, submitter email validation behavior, and step-by-step diagnostic procedures for FormMail Hub.

If you encounter persistent delivery failures after following this guide, contact technical support directly via Support & Contact. You can also test standard form submit behavior using our official Live Demo Form.

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1. Internal Delivery Status Codes & Retry Lifecycle

When a Google Form submission triggers an automated email or dynamic broadcast, FormMail Hub processes the job through its external cloud queue engine. Outbound email jobs undergo automated retries for transient errors before concluding execution.

[ Outgoing Email Queued ]
            |
            v
[ Attempt Delivery via SMTP Relay ]
            |
    +-------+-------+
    |               |
    v (Success)     v (Failure / Error)
[ Mark DELIVERED ]  |
                    v
          [ Check Remaining Attempts ]
                    |
          +---------+---------+
          |                   |
          v (More attempts)   v (Last Attempt Exhausted)
   [ Auto-Retry Delay ]   [ Record metric: emails_failed ]
   (No failure recorded   [ Send SMTP Error Alert to Owner ]
    in daily stats yet)   (Subject to 1-hour cooldown)

Status Code	Severity	Description	Queue & Retry Behavior	Recommended Remediation
DELIVERED	Info	Email successfully accepted by the downstream SMTP host.	Execution Complete.	None required.
QUEUED	Info	Message is buffered in the outbound cloud queue awaiting dispatch.	Active job in worker queue.	Allow process to finish; verify queue depth if delay exceeds 5 mins.
BOUNCED_HARD	Error	Permanent failure; recipient address does not exist or domain host rejected address.	Fails terminally; records emails_failed on final attempt.	Verify respondent email spelling; ensure email regex compliance (user@domain.com).
BOUNCED_SOFT	Warning	Temporary failure (mailbox full, greylisting, or remote host throttling).	Auto-retries across exponential backoff intervals.	If condition persists after retries are exhausted, check destination domain MX records.
FAILED_SMTP_AUTH	Error	Invalid SMTP credentials or authentication mechanism failure.	Retries exhausted; dispatches SMTP alert to Admin on final attempt.	Update SMTP username/password or generate a fresh 16-character Gmail App Password.
FAILED_SMTP_CONNECT	Error	Connection to remote SMTP host timed out or port was unreachable.	Retries up to configured max attempts before terminal fail.	Confirm host IP/domain and verify network firewall permits outbound connection on ports 587 or 465.
RATE_LIMITED	Warning	Daily sending quota or provider rate limit reached (e.g., Gmail daily quota).	Pauses dispatch; retries during next available window.	Switch sending route to Custom SMTP (Amazon SES, SendGrid) or wait for quota reset.
TEMPLATE_ERROR	Error	Dynamic tag syntax malformed or unresolvable tag reference.	Dispatch skipped to avoid rendering raw tag errors to end users.	Inspect template syntax; verify tag names match form field question labels.
OPT_OUT_SKIPPED	Info	Recipient previously unsubscribed via automated unsubscribe link.	Dispatch intentionally bypassed to maintain CAN-SPAM/GDPR compliance.	No action required. Contact is automatically suppressed.

Failure Metric Tracking Rule: The emails_failed counter in daily statistics is strictly incremented only on the final attempt after all allowed queue retry attempts have failed completely. This prevents temporary network blips from distorting daily error metrics.

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2. Submitter Email Extraction & Format Validation

To prevent dispatch errors caused by invalid respondent input, FormMail Hub enforces strict email format validation when mapping form question fields:

• Regex Validation: Mapped respondent email values must match the standard email pattern (^[^\s@]+@[^\s@]+\.[^\s@]+$).
• Invalid Data Handling: If a form response contains non-email data (e.g., phone numbers or text) in the mapped email field, the system issues a warning log and automatically falls back to the form’s default SYSTEM_EMAIL / responder_email capture.
• Dynamic Resiliency: The engine inspects both field IDs and question titles to locate valid email addresses accurately.

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3. Standard SMTP Response Codes Catalog

FormMail Hub captures raw response strings returned by custom SMTP servers (Amazon SES, SendGrid, Mailgun, custom relays) and standard Google Workspace SMTP servers (smtp.gmail.com).

2xx Success Responses

• 250 2.1.0 OK: Message accepted for delivery by the downstream mail transfer agent (MTA).
• 235 2.7.0 Authentication successful: Credentials verified successfully with the remote host.

4xx Temporary Failures

• 421 4.7.0 Try again later: Service temporarily unavailable (frequently caused by grey-listing or transient server overload).
  • Action: System auto-retries. If persistent, check destination MTA status.
• 451 4.4.0 DNS query failed: The sending relay could not resolve MX records for the recipient’s domain.
  • Action: Confirm DNS health for recipient domain.
• 452 4.2.2 Mailbox full: Recipient mailbox has exceeded storage quota.
  • Action: Soft bounce handling; notify respondent through alternative channel if critical.

5xx Permanent Failures

• 535 5.7.8 Authentication credentials invalid: SMTP username or password rejected.
  • Action: Re-authenticate credentials under Settings > SMTP Config. For Gmail/Workspace accounts, ensure 2-Step Verification is enabled and an App Password is used instead of your normal account password.
• 550 5.1.1 User unknown / Invalid recipient: Recipient address does not exist on target domain.
  • Action: Address marked invalid.
• 550 5.7.1 Message rejected due to DMARC / SPF policy: Sender domain failed domain authentication verification at recipient inbound gateway.
  • Action: Verify SPF (v=spf1 ...) and DKIM TXT/CNAME records in your DNS manager. Ensure the From header address matches your authenticated domain identity.
• 554 5.2.0 Transaction failed - Spam content detected: Remote spam filter flagged message body or subject line.
  • Action: Audit email body HTML; remove aggressive trigger words, shorteners, or unauthenticated links.

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4. Trigger & Execution System Errors

Errors occurring within the Google Workspace runtime environment or form submit trigger hooks are mapped below:

Error Key	Meaning	Primary Cause	Resolution Strategy
ERR_AUTH_SCOPE	Missing required OAuth permissions.	Workspace scopes modified or authorization revoked.	Re-open FormMail Hub add-on sidebar and complete the account re-authorization prompt.
ERR_FORM_ID_NOT_FOUND	Detached form instance.	Form container moved or copy created without resetting trigger bindings.	Re-bind FormMail Hub in the new Google Form via Settings > Re-link Form.
ERR_MAX_EXECUTION_TIME	Apps Script execution limit hit (6 minutes).	High submission spike processed synchronously.	The external cloud platform engine processes queues independently, eliminating Apps Script timeout limits.
ERR_QUOTA_EXCEEDED	Google Workspace daily email quota exhausted.	Form triggered using default Google relay without Custom SMTP.	Configure a Custom SMTP integration (e.g., Amazon SES or SendGrid) to bypass Google limits.
ERR_TAG_PARSING_FAILED	Dynamic tag evaluator failure.	Corrupted bracket syntax (e.g., {Question Name missing closing brace }).	Correct syntax in the email template editor.

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5. Audit Log Field Schema

The FormMail Hub Audit Log tracks all submission and email event data in JSON format for debugging and compliance auditing.

Log Entry Structure

{
  "eventId": "evt_987f6a5b4c",
  "timestamp": "2026-03-31T14:32:01.120Z",
  "formId": "1FAIpQLSc...",
  "submissionId": "sub_1234567890",
  "recipient": "client@example.com",
  "routeType": "CUSTOM_SMTP_SES",
  "smtpHost": "email-smtp.us-east-1.amazonaws.com",
  "status": "DELIVERED",
  "responseCode": 250,
  "responseMessage": "2.0.0 OK 0100018e...",
  "latencyMs": 342,
  "tagsEvaluated": {
    "First Name": "Jane",
    "Company": "Acme Corp"
  },
  "retryCount": 0
}

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6. Step-by-Step Diagnostic & Recovery Matrix

[ Email Not Received ]
         |
         v
Check Audit Log Status Code
         |
         +---> Status: FAILED_SMTP_AUTH  --> Go to Step A: Re-verify Credentials & App Passwords
         |
         +---> Status: BOUNCED_HARD      --> Go to Step B: Validate Recipient Email Address
         |
         +---> Status: RATE_LIMITED      --> Go to Step C: Configure Custom SMTP Provider
         |
         +---> Status: DELIVERED (Spam)  --> Go to Step D: Verify Domain SPF / DKIM / DMARC

Remediation Steps

Step A: Re-verify Credentials & App Passwords

1. Open FormMail Hub > Settings > SMTP Config.
2. If using Gmail/Workspace, confirm 2-Step Verification is active in Google Account settings.
3. Generate a brand new 16-character App Password and paste it into the password field without spaces.
4. Click Test Connection to verify response code 235 and unlock UI tabs.

Step B: Validate Recipient Email Address

1. Check the dynamic tag assigned to the recipient field (e.g., {Email Address}).
2. Verify that the form question uses Google Forms Response Validation configured to require a valid email format (Text > Email).
3. Ensure submitted values conform to standard format (user@domain.com).

Step C: Configure Custom SMTP Provider

1. Connect an external email infrastructure provider (Amazon SES, SendGrid, Mailgun) under Settings > SMTP Config.
2. Ensure production mode access is granted if using AWS SES (exit sandbox mode).

Step D: Verify Domain SPF / DKIM / DMARC

1. Run a domain check on your sending domain via a DNS diagnostic tool.
2. Confirm SPF record includes your SMTP provider:

   v=spf1 include:amazonses.com ~all

3. Ensure DKIM keys are active and DMARC record is published at _dmarc.yourdomain.com.

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7. Official Support & Resources

If technical diagnostics do not resolve your issue, access our official resources:

• Help Desk: Support & Contact
• Add-on Installation: Google Workspace Marketplace Listing
• Interactive Sandbox: Live Demo Form
• Legal Policies: Privacy Policy | Terms of Service

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Google Forms™ and Gmail™ are trademarks of Google LLC.