The ACP's Mass E-mail feature is hard-coded to include 50 recipients per E-mail. However, many ISPs/hosting companies allow fewer. This value should be configurable in the ACP and perhaps the default should be lowered (to 10). The message provided is absolutely no help in isolating the problem, either.

See the problem described in http://www.phpbb.com/community/viewtopic.php?p=12979555#p12979555 for details and a possible fix.