now I tried to render this address field header 

u'Name <abc\u03a3@xy>, abc@ewf, "Nameß" <weofij@fjeio>'

with 
h = email.Header.Header(continuation_ws='')
h.append ... / email.Header.make_header via these chunks:

[('Name <', us-ascii), ('abc\xce\xa3', utf-8), ('@xy>, abc@ewf, "', us-ascii), ('Name\xc3\x9f', utf-8), ('" <weofij@fjeio>', us-ascii)]

the outcome is:

'Name < =?utf-8?b?YWJjzqM=?= @xy>, abc@ewf, " =?utf-8?b?TmFtZcOf?=\n " <weofij@fjeio>'


(note: local part of email address can be utf too)

It seems to be impossible to avoid the erronous extra spaces from outside within that email.Header framework.
Thus I guess it was not possible up to now to decently format a beyond-ascii MIME message using the official email.Header mechanism? - even when pre-digesting things