Add References and InResponseTo Headers

[Biztactix-Ryan]

Add Headers for References and InResponseTo They were previously ignored, which prevented Office 365 correctly Threading return messages sent.

Adding Headers should be done too... But I just need these 2 headers for now. https://documentation.mailgun.com/en/latest/api-sending.html#sending Check the headers prefix documentation.

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Ryan Tregea Biztactix