get_autocrypt_header

Creates a new AutocryptHeader based on the relevant Autocrypt header associated with the sender of an e-mail message.

If the message has no sender in the From: field, or has more than one sender, then this function will return null. Autocrypt should ignore the message entirely.

If there is one sender, but no single Autocrypt header is found that matches that e-mail address, a AutocryptHeader will be returned for the sender, but it will be incomplete (see is_complete).

Note that the following types of Autocrypt headers will not be returned by this function:

• headers that do not match an address in "From:"
• unparseable headers
• headers with unknown critical attributes
• duplicate valid headers for the sender's address

The returned Autocrypt header will have its effective_date set to the earliest of either:

• the Date: header of the message or
• now (or the current time, if now is null)

Parameters:

this	a Message object.
now	a DateTime object, or null

Returns:

a new AutocryptHeader object, or null if the message should be ignored for purposes of Autocrypt.