Disable Zimbra’s duplicate mail detection

Zimbra can discard duplicates of incoming mail. This has certain advantages, but for us, where different people use the same account with different identities, this prevents a message from being delivered to multiple virtual inboxes.

To disable this, do this as user zimbra:

zmprov mcf zimbraMessageIdDedupeCacheSize 0
zmmailboxdctl restart

Unfortunately, this has the annoying problem that conversations aren’t detected for duplicates of a message. See this forum thread for more info.