You can export email addresses from a mailing list which you manage, at any time.
1) Click 'Email newsletters' on the top menu bar into the relevant
2) Click the 'Mailing lists' tab
3) Click on the title of the mailing list you wish to export
4) Click on the 'Settings' button at the top. A dropdown will show the option to 'Export CSV file'. This automatically downloads a CSV file to your computer containing all the email addresses from the list.