fix parsing message headers with addresses that need double quotes

we are using Go's net/mail to parse message headers. it can parse addresses,
and properly decodes email addresses with double quotes (e.g. " "@example.com).
however, it gives us an address without the double quotes in the localpart,
effectively an invalid address. we now have a workaround to parse such
not-quite-addresses.

for issue #199 reported by gene-hightower, thanks for reporting!

sendmail.go

@@ -201,7 +201,11 @@ binary should be setgid that group:
 						if len(addrs) != 1 {
 							log.Fatalf("only single address allowed in To header")
 						}
-						recipient = addrs[0].Address
+						addr, err := smtp.ParseNetMailAddress(addrs[0].Address)
+						if err != nil {
+							log.Fatalf("parsing address: %v", err)
+						}
+						recipient = addr.Pack(false)
 					}
 				}
 				if k == "to" {