[RSS Feed/News] Missing "Swift" file error

[XenForo]

I have error messages and thousands of errors in my log regarding this missing file...

src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mime/Headers/AbstractHeader.php
Click to expand...

Can someone please advise what this is, where I can re-download it if needed, and where to install it?
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