Ask HN: As a business, how do you authenticate yourself to your customers?

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Hey HN,

When you have direct communications (via email, SMS, phone calls) with your customers about things that typically attract fraudulent impersonators (eg bank instructions, requesting sensitive PII), what is the best way to authenticate yourself as the real company besides the regular methods (eg email with spf & dkim or website) ? Do your customers sometimes proactively request extra measures?

Thanks for your answers.