> Subaddressing is the practice of augmenting the local-part of an
    > [RFC2822] address with some 'detail' information in order to give
    > some extra meaning to that address.  One common way of encoding
    > 'detail' information into the local-part is to add a 'separator
    > character sequence', such as "+", to form a boundary between the
                           ^^^^^^^^^^^
    > 'user' (original local-part) and 'detail' sub-parts of the address,
    > much like the "@" character forms the boundary between the local-part
    > and domain.

(Highlighting by me)

The RFC even gives an example using the hash:

    > o  A message addressed to "5551212#123@example.com" is delivered to
         the voice mailbox number "123" at phone number "5551212".