Users receiving email messages 'late'

[tenpagyatso]

I've been receiving complaints from a small list of users of missing out on specific email messages. These messages are part of conversations threads containing approximately six other email addresses, all on the installation of ZCS.

What happens is that days, sometimes weeks, after these conversation threads have started they suddenly become part of the conversation threads late in the game. But the complainant's address is seen in the headers of the messages from the very beginning of these conversation threads.

Is it possible that if a user somewhere in the middle of one these conversation threads adds the complainant's email address that this email address is then added post-facto to the entire thread? Ie. Zimbra goes 'backwards in time' through the thread so that it looks as if this complainant's address has always been included in the thread?

Or is this simply a small number of users who manage their email rather poorly?