Make sure you keep copies of all correspondence, including relevant documentation.

所有的来往信函都要有备份，包括相关的证明文档。

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Make sure you keep copies of all of your financial documents, tax returns and supporting documents if you get divorced.

让你保持你的所有财务文件的副本、报税及证明文件，如果你离婚。

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Just as you keep copies of all letters in a paper file folder, you should keep copies of all letters on your computer in the directory for that client.

就像把信函的副本保存在一个纸文件夹中一样，您可以将信函的电子副本保存在电脑中专门为该客户建立的文件夹中。

youdao

Keep all receipts of any expenditure and, if appropriate, copies of telephone bills where you can identify calls made on behalf of the web business.

BBC: NEWS | Business | When does eBay selling become taxable?