[Your Name, & Address here]
[Recipient Name & address here]
I am writing in to inform you that your application for maternity leave has been accepted.
I am really happy for you and [Spouse name], and I wish the best to you.
I am really impressed that you organized your assignments well in advance and have handed over the work load to your colleagues to complete during your absence. I am sure your team will do the work in time.
Please make sure that you notify me of any kind of changes you may want for the duration of your leave.