[otrs] Can't create PID PostMasterPOP3, because it's already running

Hi, I get this message every now and then in the System log. "Can't create PID PostMasterPOP3, because it's already running (*****.***.**/25099)" The 25009 is there for an example. I always get different numbers there. I don't know what causes it, but with some experiments I found out that when this thing happens, the OTRS system cannot send any e-mail notifications. Is there any way to prevent fix this?