WordPress 5.5 compatibility issue

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WordPress 5.5 has changed and upgraded its PHPMailer class to a later release, see the release notes:

Version 5.5

The PHPMailer library just got a major update, going from version 5.2.27 to 6.1.6.

This change is causing 504 timeout errors when trying to send an email using your plugin. At the moment 5.5 is not compatible with your plugin.

Here is PHPMailer’s version 6.1.7 stable repo fyi:
https://github.com/PHPMailer/PHPMailer