Following http://www.ietf.org/rfc/rfc2821.txt every SMTP Client has to take care about the number of recipients of a mail. The server must take up to 100. The clients are not allowed to send more than the server expected.

eZ publish sends the mails to an unlimited count of recipients.
The attached patch will fix the problem.
A new setting: RecipientsLimit can be set. Now the mails will be send in given chunks.

Additionaly this bug is fixed as well in the patch:
http://ez.no/community/bugs/error_in_lib_ezutils_classes_ezsmtp_php