Someone can explain to me that this option is made

[panuweb]

How can I customize the layout of the compose email modal window

Exactly so I understand, but it happens that it is a prefixed list. The user can only add and sort items that are already predefined.

I also tried with creating an empty item and let me add it anyway.

That's why I asked

Any idea? or it is a bug?

[igrp]

It means than empty string ("") is not allowed as value - you have to enter something.

[panuweb]

Someone can explain to me that this option is made, so that I can translate it correctly

It belongs to the Entity Editor, and appears in the type fields Array