The grievance letter to an employer is a formal notice which must be served properly.
Check your contract of employment for any special rules about serving notices and make sure they are followed.
The safest way to serve notice is to deliver in person or failing this, is to send by courier, post or email.
In person: hand the letter to your employer personally.
Post: if you are serving notice by post then it is recommended that you use recorded delivery and that you keep a copy of the proof of posting and delivery.
Email: check that the employment contract allows you to send notices by email and whether it requires you to request a "read receipt" as proof of service.
However you deliver the letter we recommend you tell the employer in person before delivering the letter that you are raising a grievance and try first to have an informal chat. If this is not practical, then you must make sure that the employer has received and read the letter.