Problems with my email ID

Hello friends,
I'm trying to change my email ID from ismael@ecam.es to ismael@madridencorto.es
but the system says that it have already exist but both of them are mine (include the domain),
(i use google apps for ismael@madridencorto.es and I have tried to use your zoho projects plugin for gmail apps)
how it is possible that I cannot change my email?

thanks!

Ismael