smtplib: LMTP broken in the case of multiple RCPT

[jasenbetts]

BPO	32803
Nosy	@warsaw, @middagj
PRs	gh-76984: Handle DATA correctly for LMTP with multiple RCPT #18896

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-02-08.23:25:09.725>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'smtplib: LMTP broken in the case of multiple RCPT'
updated_at = <Date 2020-11-13.23:14:47.716>
user = 'https://bugs.python.org/jasenbetts'

bugs.python.org fields:

activity = <Date 2020-11-13.23:14:47.716>
actor = 'middag'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-02-08.23:25:09.725>
creator = 'jasen betts'
dependencies = []
files = []
hgrepos = []
issue_num = 32803
keywords = ['patch']
message_count = 4.0
messages = ['311854', '311855', '369277', '380939']
nosy_count = 3.0
nosy_names = ['barry', 'jasen betts', 'middag']
pr_nums = ['18896']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32803'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[jasenbetts]

smtplib's LMTP support is broken,

LMTP returns multiple responses at and of data if there have been multiple successful RCPT TO commands, but smtplib::data() only looks for a single response.

see the example conversation on page 3 of RFC2033

This makes LMTP unusable if there is more than one RCPT

[warsaw]

3.5 and 3.4 are in security fix only mode, so narrowing the relevant versions.

[middagj]

Could anyone take a look to the PR?

[middagj]

It would be nice if someone could take a look.