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.