kalarm should use the default smtp account ( i have only one a google smtp) but it sends the messages to the outbox of the local folders (which i cannot disable, every time i delete them they come back by themselves!) and the send fails.

How i can set kalarm to send the messages with my single and default smtp account ?

This looks like a bug to me. What's exactly the failure you're seeing?

In kmail the message remain in the local folder's outbox, instead of the gmail outbox, and it has the message:
"Failed to transport message. The server did not accept the sender address "Dimitrios Glentadakis <dglent AT gmail DOT com>". The server responded: "5.5.2 Syntax error. y12sm9384486eem.7"

I think that i have to reported to the kde bug tracker

I filled a bug report here:
https://bugs.kde.org/show_bug.cgi?id=301946