email.message.Message.senders discarding valid addresses

senders looks in message headers such as From to find sender addresses and then passes the value to email.utils.parseaddr. If the header value is broken across two or more lines (we have some stupidly long display names around here), parseaddr fails and returns an invalid address.

For example, given a header like:

From: "User, A. Very Long Name W. (CNTR-DEPT)[An Employer,\r\n
         Inc.]" <a.user-1@example.org>

parseaddr returns the value up to the CRLF, which address validation objects to.

Expect a merge request shortly.