So that’s interesting. I thought that setting `international = True` (see line https://github.com/python/cpython/blob/master/Lib/smtplib.py#L947) would be a neat workaround, but the opposite.

When delivering those emails to Gmail I started seeing 

    Failed to send email: (555, b'5.5.2 Syntax error, goodbye. s53-v6sm1864855qts.5 - gsmtp', 'foo@bar.com')

and it turns out (according to the IETF message linter, https://tools.ietf.org/tools/msglint/) that:

-----------
UNKNOWN: unknown header 'User-Agent' at line 4
ERROR: missing mandatory header 'date' lines 1-7
ERROR: missing mandatory header 'return-path' lines 1-7
OK: found part text/plain line 9
WARNING: line 13 too long (109 chars); text/plain shouldn't need folding (RFC 
         2046-4.1.1)
WARNING: line 15 too long (124 chars); text/plain shouldn't need folding (RFC 
         2046-4.1.1)
WARNING: Character set mislabelled as 'utf-8' when 'us-ascii' suffices, body 
         part ending line 22
-----------

It seems that now “Date” and “Return-Path” header entries are missing when the email is generated.

I reverted the initial change. Any updates on the multiple CR problem when flattening?