In order to export contacts from Mac Mail, the email addresses must be saved in your Contacts (a separate application). However, just because you have emailed someone before does not mean they are automatically added your Contacts.
In order to add these "remembered" email addresses to your Contacts and export them, please follow the instructions below.
1. Open the Mail application.
2. Go to Window from the Menu bar.
3. Select Previous Recipients.
4. To Select All - click on the first contact, scroll to the bottom of the window, hold down Shift key and click on the last contact.
To Select Particular Contacts - click on the first contact you want to add, scroll to the next contact you would like to add, hold down Command (or the Apple icon on older keyboards) and click on the next contact. Continue this step until all of the emails you want to add are selected.
5. Once you have made your selections, click Add to Contacts.
6. Now you are able to export these email addresses from your Contacts app.
Just refer to these instructions.
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