"You have a letter."
Translation:Du har et brev.
While it may be descriptive to call them transgender, it's a little bit comical to use that adjective in this linguistic situation.
Bok is of feminine gender which means it can be preceded by either ei or en. This implies that feminine nouns can take the masculine indefinite article en, thus lowering the number of genders to two (common [m+f] and neuter). So, the right term to use is common, not transgender.
Dike is of neuter gender so it can only come after the indefinite article et.