Takayuki,
Thank you for replying!
The error message I get is in Spanish because my WP is in that language. In the CF7 panel, would be error message #2. The translation could be: "Your message failed to be sent. Please try again later or contact the administrator by other means".
This is how I set the dropdown:
[select destinatario "Contacto General | email1@email.com"
"Experimendo San Luis | email2@email.com"
"Contrataciones | email3@email.com"]
Of course I've used real e-mail addresses.
And the "To" field is set to: [destinatario]
Thank you!