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Yes, you can, @nepw2014.
You'll want to go to the Gear icon and select Import data. From there, you'll be able to import the mailing list to your customer list. Let me guide you how.
Before doing so, ensure that the mailing list is either in an Excel or CSV format.
Here's how to import the list in your QuickBooks Online (QBO) account:
For more details about importing customer list to your QBO account, check this great article: Import customer or vendor contacts from Outlook, Excel or Gmail.
You can also check this article and read some frequently ask questions about importing data to QBO: Common questions about importing data to QuickBooks Online.
You can always get back to us anytime you need help with your QBO account. I'll be around to assist. Take care and stay healthy.