coarse-grained Date instead of 1970

Author: link2xt

src/mimefactory.rs

@@ -875,11 +875,28 @@ impl MimeFactory {
                 }
             } else if is_encrypted && header_name == "date" {
                 protected_headers.push(header.clone());
+
+                // Coarse-grained date goes to unprotected header.
+                //
+                // We cannot just send "Thu, 01 Jan 1970 00:00:00 +0000"
+                // or omit the header because GMX then fails with
+                //
+                // host mx00.emig.gmx.net[212.227.15.9] said:
+                // 554-Transaction failed
+                // 554-Reject due to policy restrictions.
+                // 554 For explanation visit https://postmaster.gmx.net/en/case?...
+                // (in reply to end of DATA command)
+                //
+                // and the explanation page says
+                // "The time information deviates too much from the actual time".
+                let coarse_grained_timestamp = self.timestamp - (self.timestamp % 1000000);
+                let unprotected_date =
+                    chrono::DateTime::<chrono::Utc>::from_timestamp(coarse_grained_timestamp, 0)
+                        .unwrap()
+                        .to_rfc2822();
                 unprotected_headers.push((
                     "Date",
-                    mail_builder::headers::HeaderType::Raw(
-                        "Thu, 01 Jan 1970 00:00:00 +0000".into(),
-                    ),
+                    mail_builder::headers::raw::Raw::new(unprotected_date).into(),
                 ));
             } else if is_encrypted {
                 protected_headers.push(header.clone());