When trying to find the email gateway address, use the
`email.util.getaddresses` function to deal with cases
where multiple recipients are included in the email header
or the stream address appears as an angle-addr with a
name given (e.g. if someone added it to their address book).
Added some other headers where the required address may
appear: "Resent" headers are sometimes used for forwarding,
and streams may also be found in CC. There is no way to find
the address if the email was recieved as a BCC.
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