What is blackholing?
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Blackholing is when a receiving server accepts your message and then silently throws it away, without ever sending a notification back.
From your side, everything looks fine. The receiver said 250 OK, your ESP logs a clean delivery, and no bounce ever comes back. No bounce, no error, no signal. That is what makes it harder to catch than a rejection. A rejection is always reported to you, because your server is on the wire when it happens. A blackholed message just disappears.
The name goes back to the first blocklist, the Realtime Blackhole List, where "blackhole" meant routing traffic from listed IPs into oblivion. Today most servers reject or filter listed mail rather than silently dropping it, but some still accept first and filter later.
You cannot see a blackholed message, so you spot it by what it leaves behind: open or click rates dropping with no content or list change, or one mailbox provider showing much lower engagement than the others. If your numbers feel off and you are not sure why, that is what the free SOS hotline is for.
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