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Outlook is a free email client from Microsoft that can be connected to your Gmail account to send out mail merges. To install Google Apps G Suite Sync for Microsoft Outlook, follow the instructions on this page: Microsoft Outlook and Google Apps G Suite Sync for Windows
There are Gmail add-ons that provide mail merge functionality. ITS does not support the use of third-party add-ons because they are not fully supported by Google and also warns that there are known malicious or insecure plugins that could steal your data or compromise your computer.
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