If your app keeps retrying the same bad addresses, you waste API calls, increase bounce rates, and make deliverability issues harder to debug. That is exactly why email suppression lists matter. They prevent future sends to recipients who already signaled “stop” through bounces, spam complaints, or unsubscribes.
This guide is for SaaS and development teams sending transactional emails like OTPs, password resets, receipts, and alerts. It explains what belongs in a suppression list, how bounces, complaints, and unsubscribes differ, and how to connect suppression logic to your sending pipeline using events and logs. You will also get a simple ruleset, a checklist-style table, and a troubleshooting flow for the common support question: “Why did this user stop receiving emails?”
To set this up in a practical way, review the MailCub docs and use the Transactional Email product to trace events and logs while implementing suppression checks.
Quick Answer
- Add recipients to suppression when they hard bounce, complain, or unsubscribe (where applicable).
- Treat complaints as highest priority and suppress immediately.
- Suppress hard bounces immediately; handle soft bounces with limited retries first.
- Enforce suppression before sending so you do not call the provider for suppressed recipients.
- Keep a controlled allowlist for verified exceptions and maintain an audit trail.
Why email suppression lists matter
Suppression lists protect both deliverability and operations. Every repeated send to an invalid address creates another failure signal you did not need. Over time, this can hurt sender reputation and make real deliverability problems harder to isolate.
They also save support time. When a user says they did not receive an email, your team can answer with evidence, such as a hard bounce on a specific date or a complaint-based suppression, instead of guessing.
Email suppression lists explained
An email suppression list is a do-not-send dataset, usually keyed by recipient and sometimes by category. Its purpose is simple: prevent repeat damage and unnecessary retries.
It is not the same as:
- A contact list (people you want to message)
- An external blocklist (reputation or DNS-based lists)
- A preference center (granular opt-down settings)
Email suppression lists for bounces, complaints, and unsubscribes
Most teams track three main suppression types, and each one needs its own handling rules:
- Bounces: delivery failed (hard or soft)
- Complaints: recipient marked the email as spam
- Unsubscribes: recipient opted out, usually for marketing or optional product messages
Keep these suppression types separate because they have different severity and different safe unsuppress rules.
Step-by-step setup for suppression lists
Step 1) Decide suppression scope
Pick one suppression scope model and document it clearly.
- Global suppression: safest for complaints and hard bounces
- Category suppression: marketing suppressed, critical transactional still allowed (use carefully)
- Tenant/app suppression: useful for multi-tenant products or multiple apps
A clear scope avoids confusion when support or engineering needs to explain why a send was blocked.
Step 2) Define a simple ruleset
Start with conservative defaults so your system is protective from day one.
- Complaint: suppress immediately, no retries
- Hard bounce: suppress immediately
- Soft bounce: retry with limits, then suppress after repeated failures
- Unsubscribe: suppress marketing and optional emails; define a clear policy for transactional categories
This ruleset is simple enough to implement quickly and safe enough to reduce repeat failures.
Step 3) Capture events and write suppression records
Suppression only works when it is driven by real signals. Use webhook callbacks, event tracking, or provider log exports to capture:
- Bounce events (including bounce type and reason)
- Complaint events
- Unsubscribe events (if supported in your flow)
Store a structured suppression record with key details such as recipient, suppression type, reason, provider message ID, and timestamp. This makes debugging and support much easier later.
Step 4) Enforce suppression before sending
This is the most important engineering control. Before you call your email API:
- Check whether the recipient is in the suppression list
- If suppressed, return a clear “suppressed” status to your app
- Log the suppression hit with type and timestamp
Doing this before sending avoids wasted API calls and keeps your metrics cleaner.
Step 5) Add a controlled allowlist for verified exceptions
Sometimes you need an override, such as QA testing, re-consent, or a verified false positive. Keep allowlisting controlled and limited.
Safe allowlist practices:
- Make overrides time-bound when possible
- Record who added the allowlist entry, why, and when
- Limit overrides to specific message categories if needed
You can use the MailCub docs to map suppression and event handling in your app flow, and review plan options on the pricing page if you are rolling this out across environments.
Step 6) Review suppression quality weekly
Suppression is not a one-time setup. A short weekly review helps you catch quality issues and false positives early.
Check these items each week:
- Top hard-bounce reasons
- Complaint spikes by template or user flow
- Unsubscribe spikes after product or messaging changes
- Allowlist usage (it should stay rare)
This keeps your suppression logic useful and trustworthy over time.
Suppression type, action, and safe unsuppress guidance
- Hard bounce: Trigger = invalid mailbox — Default action = suppress immediately — Safe unsuppress = only with strong evidence the address is valid again
- Soft bounce: Trigger = temporary failure — Default action = retry with limits — Safe unsuppress = clears after a successful send or a defined time window
- Complaint: Trigger = user reported spam — Default action = suppress immediately — Safe unsuppress = rarely, and only after explicit re-consent
- Unsubscribe: Trigger = user opted out — Default action = suppress marketing/optional email — Safe unsuppress = user re-subscribes through a confirmed flow
- Allowlist: Trigger = support override — Default action = bypass suppression — Safe use = keep time-bound and audited
Common mistakes to avoid
- Treating soft bounces like hard bounces, which suppresses valid users too aggressively.
- Retrying sends after a spam complaint, which is high risk.
- Not storing timestamps or reasons, so support cannot explain what happened.
- Enforcing suppression only after sending, which wastes calls and inflates failure stats.
- Mixing marketing unsubscribes with critical transactional emails without a clear policy.
Troubleshooting suppression issues
Problem: “Why is this user not receiving emails anymore?”
Check in this order:
- Is the recipient suppressed (bounce, complaint, or unsubscribe)?
- What is the suppression reason and timestamp?
- Was it a hard bounce or repeated soft bounces?
- If unsubscribe, what category does it apply to (marketing, optional, or critical)?
- If an override is necessary, add a time-bound allowlist and document the reason
This process gives your team a clear, evidence-based answer instead of a vague reply.
Problem: Complaints spiked suddenly
Treat this like an incident and investigate quickly:
- Isolate the template or user flow causing the spike
- Check audience or list-source quality
- Confirm your sending identity and user expectations match the content being sent
Complaint spikes usually point to a mismatch between what users expected and what they received, so fast isolation is important.
FAQ
What are email suppression lists?
Email suppression lists are do-not-send records that prevent future sends to recipients who bounced, complained, or unsubscribed. They reduce repeat failures and protect sender reputation.
Should I suppress after a soft bounce?
Usually not immediately. Soft bounces are often temporary. Use limited retries with backoff, then suppress only if failures repeat within a defined time window.
How long should I keep a hard bounce suppressed?
Hard bounces usually indicate an invalid mailbox, so suppression is often long-lived. Only remove it when you have strong evidence the address is valid again.
What should I do after a spam complaint?
Suppress immediately, then investigate the source: which template triggered it, what audience received it, and whether user expectations match the content.
Do unsubscribes apply to transactional email?
Unsubscribes usually apply to marketing or optional product emails. Critical transactional emails such as security alerts and receipts may still need to send, but your team should define this policy clearly.
How do I safely remove someone from a suppression list?
Use a controlled unsuppress or allowlist workflow with an audit trail. Only remove suppression after verified re-consent or clear evidence the address is valid, and use time-bound overrides where possible.
Conclusion
Email suppression lists are a small system with a big operational impact. They reduce wasted retries, lower repeat failure rates, and help support teams answer “what happened?” with evidence.
Make suppression event-driven, enforce it before sending, and keep allowlisting rare and audited. That is how you keep suppression protective instead of disruptive. To implement it cleanly, use the MailCub docs and wire your flow to the Transactional Email service with pre-send suppression checks in your next release.