Alternate addresses being rewritten

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Why alternate addresses in the header are rewritten to the primary address?

It is expected that alternate addresses, in particular those that appears in a user's mailAlternateAddress LDAP attribute, will be rewritten by the MTA to whatever address appears in the user's mail LDAP attribute. While this rewriting can be disabled by, say, using the noreverse channel option, this is inadvisable since the application of address rewriting is used for a variety of important purposes such as determing the proper postmaster address to use to report errors.

The correct way to avoid this sort of rewriting is to store the alternate address in a mailEquivalentAddress LDAP attribute instead of mailAlternateAddress. Unlike mailAlternateAddress, mailEquivalentAddress addresses are not rewritten by the MTA.