bugfix: when dkim-signing submitted messages, use the domain from the "message from header" instead of "smtp mail from"

dmarc verifiers will only accept a dkim signature if the domain the message From
header matches the domain of the signature (i.e. it is "aligned").

i hadn't run into this before and when testing because thunderbird sets the
"smtp mail from" to the same address as a custom "message from" header. but
other mail clients don't have to do that.

should fix issue #22

testdata/smtp/domains.conf

@@ -1,10 +1,12 @@
 Domains:
 	mox.example: nil
+	mox2.example: nil
 Accounts:
 	mjl:
 		Domain: mox.example
 		Destinations:
 			mjl@mox.example: nil
+			mjl@mox2.example: nil
 		JunkFilter:
 			Threshold: 0.9
 			Params: