Exporting Contacts From Your Apple Mail For Mac Account
In order to export contacts from Mac Mail, your contacts must also be saved in your Contacts/Address Book app. If you do not have any contacts saved there, then try logging into the the web portal for your email account and exporting contacts from there.
Open your Contacts/Address Book app.
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 and click on the next contact. Continue this step until all of the emails you want to add are selected.
From the Menu bar go to File > Export > Export vCard.
Enter a name for the file, select a location to export it to, and then click Save.
To save that vCard as .csv, use this tool. Make sure to select Gmail (CSV) and leave the other settings as is.