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The unofficial MailerSend Mail Provider module give WHMCS the ability to send emails through MailerSend transactional email service by going to Configuration > System Settings > General Settings > Mail tab > Click Configure Mail Provider and activate the MailerSend Mail Provider
After activating the MailerSend Mail Provider module for WHMCS you need to register at MailerSend website https://www.mailersend.com/ and get the API Token by navigate to Domains > Manage or Add Domain then copy the API Token after that enter the API information in the MailerSend Mail Provider module also you can test configuration before saving the API credentials.