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If you want to use XOAUTH2 authentication, you will also need to add a dependency on the league/oauth2-client and appropriate service adapters package in your composer.json, or take a look at by @decomplexity's SendOauth2 wrapper, especially if you're using Microsoft services.
Alternatively, if you're not using Composer, you can download PHPMailer as a zip file, (note that docs and examples are not included in the zip file), then copy the contents of the PHPMailer folder into one of the include_path directories specified in your PHP configuration and load each class file manually: