Both are equally good and acceptable in both formal and informal emails/situations.

The first one uses the future tense while the second one uses the present tense, implying the future. Both are perfectly fine to use.

Hello I would recommend both

These have the same meaning.

Hi Ling! Both work fine. You can use "will" as you're giving information about something that will happen in the future. You'll also often hear people say/see people write "I am on leave tomorrow" as we can use the present simple to talk about fixed future events. Hope this helps!