This is not a question of formality or 'officialness' - it's about the difference between spoken and written language. In spoken English, the two syllables 'You know' are a meaningless filler which we use to avoid silences while we're thinking. Instead of saying 'umm' or 'err', we say 'You know', or 'I mean'. You wouldn't put 'umm' or 'err' in an email, would you? Well, you shouldn't fill your emails with 'You knows' either. We need meaningless sounds to stop the gaps in our spoken language, but we don't use them in written language in the same way.

Things are certainly more flexible when they ask you to write informally, however the excessive use of " you know" is almost slang, so you should probably avoid it.