I tried to put this in the conf/log4j.properties file:
log4j.appender.LOGFILE.MaxBackupIndex=30
but then I get this message:
zimbra@mailserver:~/conf$ zmprov -v
log4j:WARN No such property [maxBackupIndex] in org.apache.log4j.DailyRollingFileAppender.
....
best regards
Ray