I'll try to explain my problem.
Running Release 6.0.5_GA_2213.UBUNTU8_64 OSE, users are using Thunderbird.
Half of the year everything was cool, after that time appeared a problem with several users - Thunderbird wrote that can't copy email to the folder "sent". "zmcontrol stop & zmcontrol start" could solve this problem but only for a couple of days.
I discover that there is a lack of memory on the server. And after reading about memory requirements i increased RAM size from 2Gb up to 8 Gb. (may be this information is useful).
After above 5-6 days server stop sending and receiving mail at all, with the same error from Thunderbird (can't copy email to the folder "sent"). This situation repeating every 5-6 days, server do the same - stopping send and receive mail. At that time it have more than 1Gb of free RAM and CPU usage at 10-20 percent (CPU is used by process "java", that also uses 16% of RAM)
Rebooting computer or making zmcontrol stop start helps, but i want to get rid of it as server stops right during the working day.