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Your sentence was struck out because you did not leave a space either before or after your double-dashes. You must leave at least one space.
Further to = to add more information/discussion to our earlier conversation, or to follow up on it ('following' in the sense of addition)
Pursuant to = acting upon ('following' in the sense of result)
'Dear all' is rather casual, but there seems to be no good, friendly alternative for office memos.
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