qmail messages?

[ACID25]

Hi

can anyone please explain me those messgages?

Sep 11 22:53:17 plesk4 qmail: 1189543997.887915 warning: trouble injecting bounce message, will try later
Sep 11 22:53:17 plesk4 qmail-queue[5533]: possible qmail-smtpd exited by timeout, reset connection or with "See http://pobox.com/~djb/docs/smtplf.html."
Sep 11 22:53:17 plesk4 qmail: 1189543997.945777 warning: trouble injecting bounce message, will try later
Sep 11 22:53:17 plesk4 qmail-queue[5534]: possible qmail-smtpd exited by timeout, reset connection or with "See http://pobox.com/~djb/docs/smtplf.html."
Sep 11 22:53:17 plesk4 qmail: 1189543997.969732 warning: trouble injecting bounce message, will try later

i cheched the website but i´m not sure, what is the reason for those messages

hope anyone can help me

THX and regards
ACID25