How do I download my customer’s email addresses? I need it for marketing.

Welcome and thanks for dropping by the Community, @marvin5.

In QuickBooks Online (QBO), there isn't a direct way to download customer's address or other information. However, you can export the Customer Contact List report to Excel instead. Then, make sure to customise it to only show the email address.

Here's how:

1. In QBO, go to the Reports menu at the left pane to get to the Standard reports page.
2. Type Customer Contact List in the search field and select it to open. 
3. Click the Customise option at the upper right.
4. In the Rows/Column drop-down, remove all the checkmarks except for Email. 
5. Hit Run report to refresh.

After that, click Export beside the Settings (gear) icon. For further guidance, consider checking out this article: Export reports, lists, and more.

Please let me know if you have any other issues or concerns in the comment section below. I'm always here to answer them. Have a great day!