Users can be exported using segments, from Users -> Segments
If you want to export all emails from system, you just need to create a segment with only 1 condition:
After adding attributes conditions and activity to the segment, choose Export Users. You can choose what attributes you want to include in the CSV file and then click Generate Export.
A file will be created and will be available within a few minutes to download under Users -> Exports.
Click Download and a CSV file with email and a field name _id will be downloaded.