Transactional emails fail in messy ways. Your API may say “accepted,” but the user still does not receive the password reset or OTP. Without a clean trail, teams lose time guessing whether the issue was DNS or authentication, throttling, content filtering, or a mailbox-side problem.
This guide explains how to build reliable delivery tracking using email delivery logs, provider message IDs, and actionable alerts. It covers a simple and practical model: correlation ID → provider message ID → log status. It also shows what to extract from logs (timestamps, status, bounce and deferral details) and how to alert on patterns like spikes, domain outages, and authentication failures.
The setup is practical for both paths: teams using webhooks and teams that can only poll logs. The goal is a delivery tracking flow you can implement quickly and trust during real incidents.
MailCub Documentation is the right place to validate the “send → message ID → email delivery logs” flow, and the Transactional Email Service page is useful for event and webhook-based tracking. You can also check MailCub Pricing when planning monitoring and alerting at higher volume.
Quick Answer
- Generate a correlation ID for every send intent (OTP, reset, receipt).
- Capture and store the provider message ID returned by the send API.
- Index email delivery logs by provider message ID and map them back to the user or action.
- Prefer webhooks for alerts, and use polling on a schedule if webhooks are not available.
- Alert on rates and trends, not single events (bounce spike, deferral spike, domain outage).
- Keep one engineer-friendly view with status, error or bounce detail, timestamps, and IDs.
Why It Matters
Delivery tracking is customer experience. If you cannot tie a support ticket to a specific message ID and log record, debugging becomes guesswork.
Logs also prevent repeat failures. Once you can see what is failing and why, you can fix root causes such as rate limits, authentication issues, and domain-level problems instead of chasing individual complaints.
Email Delivery Logs: What to Extract Every Time
Your logs should answer these questions quickly:
- Did we accept the request?
- Did the provider accept it?
- What happened after acceptance: delivered, deferred, bounced, or dropped?
- What is the exact reason and timestamp?
Minimum fields to store:
- Provider message ID
- Correlation ID (your internal ID)
- Recipient email and domain
- Template or message type
- Status and timestamps
- Error or bounce detail (raw + parsed)
MailCub Documentation can be used as the implementation reference for Email Logs and provider-side delivery status tracking.
Step-by-Step Solution
1) Create a correlation ID for each send intent
Generate a UUID for each logical email event, such as an OTP attempt, password reset request, or receipt. This becomes the join key across your application, queue, and provider logs.
Store it with:
- User ID
- Email type or template
- Recipient domain
- Environment (production or staging)
2) Capture the provider message ID at send time
When your send API returns an ID, persist it immediately. This gives you direct traceability from a user complaint to the provider log record.
Store:
- Provider message ID
- HTTP status or API outcome
- Request timestamp
3) Build the simplest data model that supports real debugging
A practical baseline schema from the source content is:
| Field | Purpose |
|---|---|
| EmailSendAttempt | Main record for one send attempt |
| correlation_id | Unique ID per send intent |
| provider_message_id | Indexed for provider log lookup |
| user_id | User-level troubleshooting |
| to_email | Recipient address |
| domain | Recipient domain grouping |
| template/type | Message category |
| created_at | Send attempt time |
| last_status | Latest normalized state |
| last_error_text | Latest failure reason |
| last_updated_at | Latest log or event update |
This supports lookup by user or email, lookup by provider message ID, and incident dashboards by time window.
4) Normalize statuses into engineer-friendly states
Providers use different status labels, so normalize them into stable internal states while still storing the original raw status.
Recommended normalized states:
- ACCEPTED (API accepted request)
- QUEUED (provider queued)
- DELIVERED
- DEFERRED (temporary issue)
- BOUNCED (hard or soft)
- DROPPED / REJECTED (policy or authentication issue)
Store both raw and normalized status side by side so engineering and support can debug accurately.
5) Alerts: webhooks first, polling second
If webhooks or events are available, use them for near real-time updates and alert triggers. This is the best path for fast detection and lower operational lag.
If you can only poll logs:
- Poll recent log changes every few minutes
- Compute rates (bounces, deferrals, failures)
- Alert on thresholds over a time window (for example, 10–30 minutes)
The Transactional Email Service is the relevant MailCub page for webhook and event-style tracking, while MailCub Documentation is the implementation reference for logs and message tracing.
6) Define alert rules that avoid noisy paging
Start with four alert types:
- Bounce spike: bounce rate above a threshold over 15 minutes
- Deferral spike: deferred rate above a threshold over 15 minutes
- Domain outage: failures concentrated on one mailbox domain (Gmail, Outlook, etc.)
- Auth/quota failure: sudden rise in 401, 403, or 429 responses
Every alert should link to:
- Example message IDs
- Top error or bounce reasons
- Affected domains
- First-seen timestamp
Common Mistakes
- Not saving provider message IDs, which removes traceability.
- No correlation ID, so support cannot map user reports to send attempts.
- Alerting on single events instead of time windows, which creates noise.
- Mixing identifiers (provider message ID vs email Message-ID header) without clear naming.
- Treating accepted as delivered and closing incidents too early.
Troubleshooting
API says accepted, but the user did not receive it
Check logs first:
- Is it still QUEUED or DEFERRED?
- Did it BOUNCE with a reason?
- Is delivery delayed for one domain only?
Action guide:
- If deferred: slow per-domain sending and apply retry backoff
- If bounced: fix addressing, authentication, or policy issue
- If domain-only: treat it as a domain incident, alert, and throttle
Deferred for a long time
Long deferrals often indicate throttling or temporary recipient-side limits.
Actions:
- Reduce concurrency
- Implement per-domain pacing
- Cap retries and escalate using log evidence
Bounce rate suddenly jumped
Group the incident by:
- Domain
- Template or message type
- Sending IP or account
- Time window
Common causes from the source content include authentication or DNS changes, compromised account behavior, content pattern changes, and provider policy updates. Use sample message IDs to reproduce and share with support teams.
MailCub Documentation is useful here for reviewing delivery log entries and message status details during incident response.
FAQ
What are email delivery logs?
They are provider records that show what happened to a message after send, including status, timestamps, and bounce or deferral details.
What’s the difference between provider message ID and the email Message-ID header?
Provider message IDs are used for provider API and log lookups. The email Message-ID header is part of the email format. They may not match, so store both if available.
If my API returns accepted, is the email delivered?
Not necessarily. Accepted usually means queued or accepted for processing. Confirm the final outcome using logs or events.
What should I alert on first for transactional email?
Start with bounce spikes, deferral spikes, domain outages, and sudden authentication or quota errors (401, 403, 429).
How often should I poll logs if I don’t have webhooks?
Start with every 5–10 minutes for recent windows, then tune based on volume and how quickly you need detection.
What’s the minimum data model for delivery tracking?
At minimum: correlation ID, provider message ID, user or recipient email, template or type, timestamps, normalized status, and error or bounce detail.
Conclusion
Tracking email delivery is an observability problem. If you capture correlation IDs, store provider message IDs, and convert logs or events into trend-based alerts, your team will debug faster and reduce repeat incidents.
Use MailCub Documentation to implement the message ID and log tracking flow, and use the Transactional Email Service for event and webhook-style delivery tracking. Review MailCub Pricing when you need to scale your monitoring and alerting setup.