class ActionMailer::Base

for the same header.
+nil+ in order to reset the value otherwise another field will be added
If you want to replace any header which already exists, first set it to

X-Anything can appear multiple times.
Fields can only appear once in email headers while other fields such as

* +references+
* +message-id+
* +orig-date+
* +reply-to+
* +bcc+
* +cc+
* +to+
* +from+
* +sender+
* +subject+

Note about replacing already defined headers:

X-Special-Domain-Specific-Header: SecretValue

The resulting +Mail::Message+ will have the following in its header:

'In-Reply-To' => incoming.message_id
headers 'X-Special-Domain-Specific-Header' => "SecretValue",

which will then be set on the +Mail::Message+ object:
You can also pass a hash into headers of header field names and values,

headers['X-Special-Domain-Specific-Header'] = "SecretValue"

object which will add them to itself.
Allows you to pass random and unusual headers to the new +Mail::Message+

def headers(args = nil)
  if args
    @_message.headers(args)
  else
    @_message
  end
end