This error occured when the email field "From" was demanded:

  File "/home/user/processing/Test/process_email.py", line 84, in __init__
    self.field_from_full = msg.get("From")
  File "/usr/local/lib/python3.6/email/message.py", line 471, in get
    return self.policy.header_fetch_parse(k, v)
  File "/usr/local/lib/python3.6/email/policy.py", line 162, in header_fetch_parse
    return self.header_factory(name, value)
  File "/usr/local/lib/python3.6/email/headerregistry.py", line 586, in __call__
    return self[name](name, value)
  File "/usr/local/lib/python3.6/email/headerregistry.py", line 197, in __new__
    cls.parse(value, kwds)
  File "/usr/local/lib/python3.6/email/headerregistry.py", line 337, in parse
    kwds['parse_tree'] = address_list = cls.value_parser(value)
  File "/usr/local/lib/python3.6/email/headerregistry.py", line 328, in value_parser
    address_list, value = parser.get_address_list(value)
  File "/usr/local/lib/python3.6/email/_header_value_parser.py", line 2336, in get_address_list
    token, value = get_address(value)
  File "/usr/local/lib/python3.6/email/_header_value_parser.py", line 2313, in get_address
    token, value = get_group(value)
  File "/usr/local/lib/python3.6/email/_header_value_parser.py", line 2284, in get_group
    if value[0] != ';':
IndexError: string index out of range

I used EmailPolicy, so the message object was instance of EmailMessage.
Header field "From" had following form: 
"From: Bonifac Karaka : bonikar@gmail.com"

I added 2 lines into the file "email/_header_value_parser.py"
into the function "get_mailbox_list(value)" before return value:

    if not value:
        value = ';'
    return mailbox_list, value


It is in attachments.

Submitted pull request: 
fix-issue-31455: Changed by adding two lines in the module "email/_header_value_parser.py" into the function "get_mailbox_list(value)" before return mailbox_list, value to avoid IndexError.

I can't reproduce this issue on the latest master branch. This seems to be fixed as a part of a different PR I suppose.

    >>> import email
    >>> msg = email.message_from_string("From: Bonifac Karaka : bonikar@gmail.com")
    >>> msg['From']
    'Bonifac Karaka : bonikar@gmail.com'

This is very similar to bpo-32178, which also is now fixed.

@david, @barry, I think we can close this issue.

Note that the reporter indicated that the message was an instance of EmailMessage (the new API).  You'd need to use policy-default to get that using message_from_string.  But yes, this was fixed in another issue.