Question to entries in logfile

[Thomas Booms]

Hi all,

what does this line means?

I have much of these in my logfiles. But I don't know why. How can I
solve this problem?

May  9 20:28:38 webserver02 qmail: 989432918.700871 warning: trouble
opening local/15/768123; will try again later

Thanks for your help!

Thomas