Apply zimbraPrefPop3DownloadSince for COS!

[zibra]

Dear zimbra team,
Following in forum, i see that zimbraPrefPop3DownloadSince attribute is set for each user, not COS. I have tried to set this attribute to COS, everything is ok and it apply for all users in this COS. However, when the users want to download the old mails, they can't set "Allow all mail, including old mail, to be downloaded" feature from zimbra web client. Do you have any ways to solve the issue or how to delete this attribute from COS.

Best regards,