Improve handling of multiple / no From addresses (#3667)

* Treat multiple From addresses as if there was no From: addr

* changelog

* Don't send invalid emails through the whole receive_imf pipeline

Instead, directly create a trash entry for them.

* Don't create trash entries for randomly generated Message-Id's

* clippy

* fix typo

Co-authored-by: link2xt <link2xt@testrun.org>

src/tools.rs

@@ -670,6 +670,18 @@ pub(crate) fn parse_receive_headers(headers: &Headers) -> String {
         .join("\n")
 }
 
+/// If `collection` contains exactly one element, return this element.
+/// Otherwise, return None.
+pub(crate) fn single_value<T>(collection: impl IntoIterator<Item = T>) -> Option<T> {
+    let mut iter = collection.into_iter();
+    if let Some(value) = iter.next() {
+        if iter.next().is_none() {
+            return Some(value);
+        }
+    }
+    None
+}
+
 #[cfg(test)]
 mod tests {
     #![allow(clippy::indexing_slicing)]