How to Handle Bounces and Suppression Lists
Sending to bad addresses burns your reputation. Here is how to handle bounces and suppression lists so your transactional email stays trusted and reaches real inboxes.
Every time you send to a dead address, mailbox providers notice, and your reputation drops a little. Do it enough and your good mail starts landing in spam. The fix is a suppression list: a record of addresses you must never send to again, updated automatically from bounce and complaint events. If you handle bounces properly and check the suppression list before every send, you protect the reputation that keeps your real mail in the inbox. Here is how to do it right.
What is a suppression list and why does it matter
A suppression list is the set of addresses your system refuses to send to. It exists because sending to bad addresses is one of the fastest ways to damage your sending reputation. Providers read repeated attempts to dead mailboxes as a sign you do not manage your list, which is exactly what spammers fail to do.
The list holds two main categories: addresses that hard bounced, meaning permanent delivery failure, and addresses that complained, meaning the person marked you as spam. Both are addresses you must stop mailing. Continuing to send to either one actively hurts you, and it can hurt the deliverability metrics that decide where all your mail lands.
How do I tell hard bounces from soft bounces
This distinction drives everything, so get it right.
A hard bounce is permanent. The address does not exist, the domain does not exist, or the server permanently rejected the mail. Suppress hard bounces immediately and permanently. There is no point retrying; the address is gone.
A soft bounce is temporary. The mailbox is full, the server is briefly unavailable, or the message was greylisted. Do not suppress on a single soft bounce. Retry with backoff. But watch for patterns: an address that soft bounces repeatedly over days is effectively dead, so most systems suppress after a threshold of consecutive soft bounces.
Getting this wrong in either direction costs you. Suppress too aggressively and you stop mailing valid users over a temporary hiccup. Suppress too loosely and you keep hammering dead addresses. A good provider classifies bounces for you and applies sensible defaults, which is one of the things worth checking when you evaluate an email vendor.
How should the suppression list actually work
Automate it end to end. Manual list management does not scale and always drifts.
Feed it from webhooks. Your provider posts bounce and complaint events, and your handler adds the address to the suppression list the moment an event arrives. This is the practical payoff of wiring up email event webhooks.
Check it before every send. Your sending path queries the suppression list first and skips any suppressed address. This has to be a hard gate, not an afterthought, because the whole point is to never send to these addresses again.
Keep the provider list and your list in sync. Most platforms maintain their own suppression list and will refuse to send to addresses on it. Mirror that in your own database so your product knows the address is bad and can prompt the user to update it, rather than silently failing forever.
A transactional platform like Usermails maintains suppression automatically from bounce and complaint data, so the dangerous addresses are blocked even if a bug in your code tries to send to one.
What about transactional mail specifically
Transactional mail has a twist: sometimes you genuinely need to reach a suppressed address, like when a user with a previously bounced email tries to reset their password after fixing their mailbox. Handle this deliberately. Allow a narrow, logged path for critical transactional messages to re-attempt a suppressed address, and re-suppress instantly if it bounces again. Do not turn this into a loophole that mails your whole suppression list.
Also make it easy for users to correct a bad address in your product. A suppressed address usually means the user needs to give you a new one. Surface that in the UI instead of letting the account rot in silence.
The habit that keeps you healthy
Classify bounces correctly, suppress hard bounces and complaints automatically from webhook events, check the list before every send, and give users a way to fix a bad address. Do that consistently and your list stays clean, your reputation stays high, and your transactional mail keeps reaching the real inboxes that matter. Neglect it and you will spend a quarter digging out of a spam-folder hole you dug one dead address at a time.