Unfortunately you can’t import a folder from a backup. You need to restore the backup, but this will replace everything in eM Client with what is in the backup.
There is a workaround:
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Backup eM Client.
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Disconnect from the Internet and restore the backup that has your deleted folder.
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Export that folder using Menu > File > Export > Export to eml files. Then select the folder you want to export.
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Restore the backup you made in 1 above and then reconnect to the Internet.
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Import the folder that you exported in 3 above using Menu > File > Import > Email (.eml).