Mail - cannot set "From" address

To override the "From" address for OS X Mail, the help says the following:

To choose an account for a message you’re composing, click the From pop-up menu and choose an email address.

If the From pop-up menu is not displayed, click the Customize pop-up menu in the header area and choose Customize. Select the checkbox next to From and then click OK.​

The trouble is that when I follow these instructions, there is no "From".

What am I doing wrong?

 

We resolved this problem today. It was caused by having 4 accounts set up, but for some reason they all had the same email address. Of course, this makes no sense, and I think Mail simply got "confused". The result was that the "From label" and its associated control did not appear, and the server control was slid over to the left to where the "From" should have appeared.

After putting the correct email addresses in, the "From" control appears as it should.