Is a business letter written in an active or passive voice?

Active.

Compare these sentences:

I will meet you at the conference at 6:00pm -- active sentence.

You will be met at the conference at 6:00pm. -- passive sentence.

In the first sentence you can see who does the action of meet.

Passive sentences don't usually say who or what does the action of the verb. So they are used when the doer of the action is not important. For example in writing that describes a process eg

Tea is picked

and then it is taken

to large sheds to dry.

(The passive verbs are in bold

).

Or:

The Mona Lisa was painted

in the 16th century.