upgrade deps; rewrite smtp session

vendor/github.com/emersion/go-msgauth/dkim/dkim.go

@@ -1,4 +1,17 @@
 // Package dkim creates and verifies DKIM signatures, as specified in RFC 6376.
+//
+// # FAQ
+//
+// Why can't I verify a [net/mail.Message] directly? A [net/mail.Message]
+// header is already parsed, and whitespace characters (especially continuation
+// lines) are removed. Thus, the signature computed from the parsed header is
+// not the same as the one computed from the raw header.
+//
+// How can I publish my public key? You have to add a TXT record to your DNS
+// zone. See [RFC 6376 appendix C]. You can use the dkim-keygen tool included
+// in go-msgauth to generate the key and the TXT record.
+//
+// [RFC 6376 appendix C]: https://tools.ietf.org/html/rfc6376#appendix-C
 package dkim
 
 import (