[Exim] Fork failed

Ups, I just upgraded to kernel 2.4.24. But noe with exim 4.20 _and_ 4.30
unfortunately on one system I get

421 <servernam> fork failed

when I telnet to 25.

"Panic" says: accept process fork failed: Resource temporarily unavailable

This is quite urgent, can anybody give a hint (quit fast....)

Thanks in advance
d.