Plus Addressing is a feature that allows a sender to append a tag to the user id of a recipient. This lets the recipient more easily sort the incoming mail using a mail filter.
For example, email@example.com could also be reached using the address firstname.lastname@example.org.
To enable Plus Addressing with Zimbra, use zmprov as follows:
zmprov mcf zimbraMtaRecipientDelimiter +
Note that the delimiter can be a character other than '+'.
To disable Plus Addressing, use zmprov as follows:
zmprov mcf -- -zimbraMtaRecipientDelimiter +
After making changes to enable or disable this feature, it may be required to restart tomcat. Postfix should pick up the changes automatically, though it may take a couple minutes to register.