unable to create domain with"ztel" name

Hi,

Few days ago i tried to create new domin with name "ZTEL" but SAP did not allow me to create domain and was asking for ACCESS key.

I was trying to create new domin with 'Z' prefix  and also i have checked that if ZTEL is not already created or not . but it was not existing object.

Note : i am able to create Data element with "ZTEL" name

Though i changed domain name to "ZTELL" , due to lack of time and did not go deeper in problem at that time.

but now want to know reason behind this behavior.  I wonder why SAP is not allowing me to create domain with name "ZTEL".

Is it reserve for something ?

Thanks in adavance