I had this issue with one user. It didn't matter what browser was being used. It was due to the fact that they were checking an external mail server for another account, and there's some aspect of Zimbra that will not show share options (Accept/Decline buttons) if a folder is also used by another mail account (non-Zimbra, like Gmail via POP3).
When you move the Share Notification e-mail to a new folder, the buttons will show up ... it was an instant fix for us. Create a new folder, and drag the Share Notification e-mail into it, and it should work. |