The Outlook plugin uses your outlook address book. This works with your local contacts and any shared distribution lists, or contacts on the network. To choose contacts, click the 'To' button.
If you want to import contacts from your ClickSend dashboard, you can export them then import into Outlook. These steps explain how to do this: https://support.office.com/en-us/article/Import-contacts-from-an-Excel-spreadsheet-to-Outlook-f6483469-0684-4c30-82c5-c881930f5451
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