1. Move (or copy) all mails into one folder
2. Click on File -> Import/Export -> Export into a file -> Comma seperated values (Windows)
3. Choose the folder which holds the mails
4. Choose export file location
5. Enable the action export emails from folder
6. Click on “map fields”
7. Click on “delete mappings”
8. From the left window choose “From (Adress)” and drag it to the right
9. Click on “OK”, then on “Finish”.

Now you should have a text file with all the eMail addresses.

An alternative way is to use a macro to add those addresses as contacts. Details here (german only).

Further links:

• Extract addresses from Outlook Mails
• Add addresses from Outlook Mails  as contacts