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Same problem here. Small mail host on my own domain.

Everything is set up perfectly, mail-tester.com gives it 10/10, and last week Gmail suddenly starts hard bouncing emails for one day and delivers to spam folder after that. Three days later everything is back to normal.

How? Why?

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Most likely, activity on your IPs or IP blocks you can't see. Spam filters use a kind of 'neighbourhood reputation' as well as info about specific domains.

Unfortunately, the case where you have one good but very low volume sender surrounded on all sides by high volume spammers is very difficult to handle for spam filters, no matter how well designed they are. And, programmers who self host mail on cheap VPS systems are by far the most likely to encounter this problem.

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