How Can I Address HR Poaching Our Trained Employees Without Our Consent?

Dear Buddies,

Can anyone guide me to draft a mail to be sent to the customer HR of my company for recruiting our employees? The situation is as follows: every year, we receive projects or work from the company. Once the project is completed, the company's HR department directly recruits our employees without our knowledge. We wish to file a complaint with the head of HR as we are a small company that invests in training our employees, only for them to be easily hired away.

Regards,
Ranganathan L

Instead of sending an email, why not have a personal discussion. Have facts and figures and your questions for recruitment. Sending an email is fine and is a record, but many nuances and points are better covered in verbal discussion.