Question 1

What is an email suppression list and why do I need one?

Accepted Answer

An email suppression list is the set of addresses your platform refuses to send to, regardless of whether your campaign segment includes them. Suppressions exist to **protect your sender reputation and stay legally compliant**: sending to a hard-bounced address damages your IP reputation; sending to someone who unsubscribed violates CAN-SPAM, GDPR, and CASL; sending to a spam complainant amplifies the original complaint at the mailbox provider. Without robust suppression handling, every operational mistake compounds — a single bad campaign can produce thousands of bounces that then keep producing reputation damage on subsequent sends. The Inbox OSS suppression engine enforces seven distinct suppression categories automatically and blocks sends at the API gateway before any reputation damage can occur.

Question 2

What are the seven suppression categories Inbox OSS tracks?

Accepted Answer

The seven categories are: (1) **hard bounces** — permanently failed deliveries; (2) **soft bounces** — transient failures that escalate to hard after three consecutive failures (the three-strike rule); (3) **spam complaints** — feedback-loop complaints from Gmail, Yahoo, Outlook, AOL, and other FBL participants; (4) **unsubscribes** — one-click and preference-center opt-outs; (5) **invalid addresses** — pre-suppressed by the validation engine before the send attempt; (6) **blocks** — mailbox-provider blocks (e.g., 550 5.7.1 rejection from a recipient policy); (7) **global suppressions** — account-wide block list independent of any campaign or group. Each category has independent retention policies and clearance permissions, so an unsubscribe can be cleared by the subscriber re-opting-in while a spam complaint stays permanent.

Question 3

How does hard-bounce suppression work?

Accepted Answer

When the recipient mail server returns a **5xx permanent SMTP failure** (mailbox does not exist, domain rejects mail, sender blocked), the address is recorded against your account suppression list immediately and surfaced via the bounced webhook event. Subsequent sends to that address are blocked at the /api/v1/mail/send gateway with a structured response indicating the suppression cause. Hard bounces persist by default but can be cleared via the dashboard or API if you have verified the address corrected — useful when an enterprise customer fixes a misconfigured mail server. The dashboard message log shows the full SMTP conversation including the exact response code and reason text so you can diagnose why each address bounced.

Question 4

What is the three-strike soft-bounce rule?

Accepted Answer

A soft bounce is a transient SMTP failure — mailbox full, server temporarily unavailable, greylisted, etc. — that the retry system can typically recover from. We retry with exponential backoff for up to 72 hours per failed delivery. If the same address soft-bounces on three consecutive sends within a 14-day window, the suppression engine **promotes the address to a hard suppression** because sustained soft-bouncing is a strong signal that the mailbox is effectively non-deliverable (abandoned, on permanent vacation, full quota indefinitely). The three-strike rule is configurable per account; some senders prefer a stricter two-strike rule, some a more lenient five-strike rule. The default of three matches industry best practice from MAAWG.

Question 5

How are spam complaints routed and handled?

Accepted Answer

Spam complaints arrive through **feedback loops (FBLs)** with the major mailbox providers — Gmail, Yahoo, Outlook, AOL, Comcast, and others. When a recipient clicks "report spam" in their mailbox UI, the provider forwards the complaint to our FBL endpoint with the original message headers. The suppression engine parses the original message ID, identifies the sender, and records the suppression against the complaining address. **Spam-complaint suppressions cannot be cleared automatically** — even via API — because the legal and reputation cost of repeating a complaint is severe. Dashboard admins with the compliance scope can manually clear individual complaints with an audit-logged justification, but this should be rare. Spam-complaint rate above 0.1 percent is the industry warning threshold and triggers a dashboard alert.

Question 6

How does unsubscribe management work?

Accepted Answer

Inbox OSS implements **RFC 8058 one-click unsubscribe** via the List-Unsubscribe-Post header that Gmail and Yahoo now require for senders over 5,000 daily messages. Every campaign and template includes the header automatically; the dashboard hosts a preference center page on your tracking domain that handles both one-click and full preference management. Subscribers can opt out of specific unsubscribe groups (newsletters, product updates, promotional offers) or globally. Group-level unsubscribes apply to every campaign in that group; global unsubscribes block all marketing email account-wide. Transactional sends (password resets, receipts) bypass the unsubscribe list by default because they are legally exempt — but the dashboard surfaces a manual block-transactional-too option for compliance edge cases.

Question 7

Can I import an existing suppression list from SendGrid, Mailgun, or another provider?

Accepted Answer

Yes. The dashboard's **Bulk Import** tool accepts CSV files containing addresses and categories — drop in your existing suppression export and the engine deduplicates, normalizes, and persists every entry. We support direct migration from SendGrid (their CSV format is auto-detected), Mailgun (TXT or CSV), Postmark, Amazon SES (the SES suppression list JSON format), and Resend. For programmatic import, the /api/v1/suppressions/bulk API endpoint accepts up to 100,000 addresses per request with idempotent retries. Bringing your suppression history with you on migration is critical — without it you start over with no protection against historically bouncing or complaining addresses.

Question 8

Can I export my suppression list for backup or migration?

Accepted Answer

Yes. The dashboard supports CSV and JSON export for any subset of suppressions (filter by category, date range, address pattern, etc.) and the /api/v1/suppressions/export API endpoint produces a streaming download for very large lists. Suppression data is portable in both directions — you own it, and we make it easy to take with you if you ever leave. We also offer a webhook firehose mode that streams every suppression event (add/remove/promote) to your endpoint in real time, useful for keeping a downstream system synced with the canonical suppression state.

Question 9

How does the validation engine integrate with suppressions?

Accepted Answer

The **7-check email validation engine** can be configured as a pre-send hook that runs on every /api/v1/mail/send call. Addresses that fail validation (no MX, syntactically invalid, known disposable, etc.) are automatically suppressed and the original send returns a structured error indicating which addresses were rejected. This catches invalid addresses **before they ever produce a bounce**, which is meaningfully better than reactive bounce-based suppression. The pre-send hook adds sub-300ms p95 latency to /mail/send calls and is included in every paid plan. For higher-throughput workflows, you can run validation asynchronously on list import instead of inline on send.

Question 10

How does this compare to SendGrid Suppression and Mailgun Bounces?

Accepted Answer

SendGrid and Mailgun both maintain hard-bounce and unsubscribe lists at production quality. Inbox OSS differs structurally on three points: (1) **seven suppression categories with independent retention and clearance policies** (SendGrid combines bounces and blocks into one category, hiding the underlying difference); (2) **three-strike soft-to-hard promotion is built-in** (Mailgun does not promote at all; SendGrid promotes inconsistently); (3) **import/export tooling supports direct migration from every major competitor** with format auto-detection. Pricing is also bundled — there are no separate charges for suppression-list capacity or API calls, even at high volume.

Never send to the wrong address twice

Hard bounces

Spam reports

Unsubscribes

Invalid

Blocks

Global suppressions

Frequently asked questions about email suppressions

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