I need an expert administrator to trouble an email issue

I require a systems administrator to troubleshoot an issue where duplicate emails are being fired from our website.

The issue could be do with a bug on any of the following:

Dedicated Windows Server which the site is on.
(the server, configured for mail.samedaydispatch.co.uk is responsible for routing all emails to O365 Exchange)

Code on site (although this has been checked many times)
The code is written in core PHP (optimised for v5.6.40).

Please do not apply unless you are proficient with Windows Server 2012 R2 administration, Core PHP and O365 Exchange.

The amount of £100 will be paid when the issue has been successfully resolved.