It depends on which you choose the transfer way.
6-digit key Transfer
Instead of keeping files on the server, files will be sent to the other party instantly.
Files uploaded to a share link are kept on the server for up to 48 hours. However, Send Anywhere Email Add-on subscribers can set a download expiration date, so files are kept on the server for the downloadable period you set. Once you delete the link or the link expires, all the files get deleted from the server for security reasons. After then you cannot retrieve anymore.
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To Device Directly
Files sent to a device are kept on the server for up to 48 hours. When the other party receives the file, the file is deleted automatically from the server and will be deleted from the server after 48 hours, even if the file is not received.
※ All ways except share-link are 1:1 one-time transfers.