[issue24066] send_message should take all the addresses in the To: header into account

[Ned Deily]

Changes by Ned Deily <nad at acm.org>:


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nosy: +barry, r.david.murray -ned.deily
stage:  -> patch review
versions: +Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue24066>
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