Re: 'why', I would assume it's from a zealous reading of RFC 2822; in its grammar a dot is defined as a separator but a separator has no defined lexical meaning for the local part of an address. A server can do whatever it wants — to the extent that 2822's predecessor, 822, received an official amendment to clarify that the local part should not be modified when forwarding messages. Prior to that it was valid for server A to remove or add dots as it felt fit, then pass that along to server B assuming it made no difference there either.
Biting the hand that feeds IT
