Math problem, 4 emails, 3 mailboxes, each mailbox must have at least 1 letters! Vote for all four letters! How many casting methods are there?

1, 2, 3, 4 represents four emails.

A, B and C represent three mailboxes.

There must be a mailbox with two emails, and the combination of two emails is (1, 2), (1, 3), (1, 4), (2,3), (2,4) and (3,4).

When one of them is selected to put two emails, there are two other ways to put the other two mailboxes, namely 6*2= 12.

* * * There are three kinds of mailboxes, 12*3=36 kinds.