E-Newsletter: SMTP Error: Data not accepted. Error

Hi,
I try to send a newsletter today.
My group is about 640 people.
The system send it correctly to about 500 people.
140 are in waiting status.

I removed all of them from the wp_enewsletter_send_members table to continue.
Now, each time I try to send it, i got an error message.

SMTP Error: Data not accepted. Error

Any idea ?

Thank you
Dimitri