[clamav-win32] Phishing caught on outbound mail but not on i…

I've noticed many times in the last few days, that an arriving phishing
attempt is not caught by ClamAV. But when I forward that same phishing
attempt as an attachment to another e-mail-address only a few minutes
later, ClamAV blocks it, e.g. with the message 550 contains virus --
(Phishing.Heuristics.Email.SpoofedDomain FOUND)

I can accept that ClamAV blocks my forwarding, but why wasn't the mail
blocked at arrival?

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