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. You can choose what attributes you want to include in the CSV file by clicking on 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.