How do they decide when to suppress an address?
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Think about what happens when you send an email and it bounces back. The question your ESP is really asking is: "Is this address gone for good, or just having a bad day?" The answer determines whether that address gets suppressed right away, after a few failed attempts, or not at all.
Some addresses trigger suppression immediately. A hard bounce means the address doesn't exist or the domain is invalid. SMTP codes like 550 or 5.1.1 are the server's way of saying "this person is not here." Disabled or terminated accounts fall into the same category. There's no point retrying those. Suppress immediately, move on.
Soft bounces are trickier. A full mailbox today might be empty tomorrow. A temporary server error might clear up in a few hours. So instead of suppressing on the first failure, ESPs watch for a pattern. The typical threshold is somewhere between 3 and 7 consecutive failures over a window of 7 to 30 days. If an address keeps failing across that window, the system treats it as effectively dead and suppresses it.
Some situations call for more caution before suppressing. Block bounces, for example, might mean your IP or domain has a reputation issue on that particular receiving server, not that the address itself is bad. Ambiguous response codes (ones that could mean several things) get held for further investigation. And if a subscriber has opened or clicked recently, that recent engagement acts as a reason to pause before suppressing. (It's a real signal that someone is there, even if the last send bounced.)
Beyond permanent suppression, there are softer options. Temporary suppression means "don't contact for now, but retry after a cooling-off period." Campaign-specific suppression means the address sits out a single send but stays in the list otherwise. These graduated levels let senders protect deliverability without throwing away addresses that might still be good.
The underlying logic is always the same trade-off. Suppress too aggressively and you lose real subscribers. Suppress too slowly and you keep hammering addresses that are hurting your sender reputation. Most ESPs land somewhere in the middle, but the exact thresholds vary, and some let you tune them for your list.
If you're seeing unusual bounce patterns and want to understand what your ESP is actually suppressing, the free Email Header Analyzer can help you read what the server actually sent back. Or if things feel off and you're not sure where to start, our SOS hotline is free.
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