diff --git a/backend/tests/check_email.rs b/backend/tests/check_email.rs
index 8679ec323..cf6278bc3 100644
--- a/backend/tests/check_email.rs
+++ b/backend/tests/check_email.rs
@@ -27,26 +27,28 @@ const FOO_BAR_BAZ_RESPONSE: &str = r#"{"input":"foo@bar.baz","is_reachable":"inv
 
 #[tokio::test]
 async fn test_input_foo_bar() {
-	env::remove_var("RCH_HEADER_SECRET");
+	env::set_var("RCH_HEADER_SECRET", "foobar");
 
 	let resp = request()
 		.path("/v0/check_email")
 		.method("POST")
+		.header(REACHER_SECRET_HEADER, "foobar")
 		.json(&serde_json::from_str::<EndpointRequest>(r#"{"to_email": "foo@bar"}"#).unwrap())
 		.reply(&create_routes(None))
 		.await;
 
-	assert_eq!(resp.status(), StatusCode::OK);
+	assert_eq!(resp.status(), StatusCode::OK, "{:?}", resp.body());
 	assert_eq!(resp.body(), FOO_BAR_RESPONSE);
 }
 
 #[tokio::test]
 async fn test_input_foo_bar_baz() {
-	env::remove_var("RCH_HEADER_SECRET");
+	env::set_var("RCH_HEADER_SECRET", "foobar");
 
 	let resp = request()
 		.path("/v0/check_email")
 		.method("POST")
+		.header(REACHER_SECRET_HEADER, "foobar")
 		.json(&serde_json::from_str::<EndpointRequest>(r#"{"to_email": "foo@bar.baz"}"#).unwrap())
 		.reply(&create_routes(None))
 		.await;
@@ -66,7 +68,7 @@ async fn test_reacher_secret_missing_header() {
 		.reply(&create_routes(None))
 		.await;
 
-	assert_eq!(resp.status(), StatusCode::BAD_REQUEST);
+	assert_eq!(resp.status(), StatusCode::BAD_REQUEST, "{:?}", resp.body());
 	assert_eq!(resp.body(), r#"Missing request header "x-reacher-secret""#);
 }
 
@@ -82,7 +84,7 @@ async fn test_reacher_secret_wrong_secret() {
 		.reply(&create_routes(None))
 		.await;
 
-	assert_eq!(resp.status(), StatusCode::BAD_REQUEST);
+	assert_eq!(resp.status(), StatusCode::BAD_REQUEST, "{:?}", resp.body());
 	assert_eq!(resp.body(), r#"Invalid request header "x-reacher-secret""#);
 }
 
@@ -98,6 +100,6 @@ async fn test_reacher_secret_correct_secret() {
 		.reply(&create_routes(None))
 		.await;
 
-	assert_eq!(resp.status(), StatusCode::OK);
+	assert_eq!(resp.status(), StatusCode::OK, "{:?}", resp.body());
 	assert_eq!(resp.body(), FOO_BAR_RESPONSE);
 }
diff --git a/core/src/smtp/parser.rs b/core/src/smtp/parser.rs
index 337cc4ab0..e1f25a312 100644
--- a/core/src/smtp/parser.rs
+++ b/core/src/smtp/parser.rs
@@ -85,6 +85,12 @@ pub fn is_invalid(e: &str, email: &EmailAddress) -> bool {
 		|| e.contains("no longer available")
 		// permanent: RCPT (<EMAIL>) dosn't exist (on @hgy.ooo)
 		|| e.contains("dosn't exist") // sic! typo is intentional
+		// 5.1.1 <EMAIL>: Email address could not be found, or was misspelled (G8) (on biotech-calendar.com, invoicefactoring.com)
+		|| e.contains("could not be found") 
+		// No such person at this address (on aconsa.com.mx)
+		|| e.contains("no such person")
+		// Callout verification failed: 550 No Such User Here (on medipro.co.uk)
+		|| e.contains("callout verification failed")
 }
 
 /// Check that the mailbox has a full inbox.
@@ -94,9 +100,9 @@ pub fn is_full_inbox(e: &str) -> bool {
 	|| e.contains("mailbox full")
 	// https://answers.microsoft.com/en-us/outlook_com/forum/all/how-do-i-interpret-the-delivery-failure-message/2f1bf9c0-8b03-4f8f-aacc-5f6ba60a73f3
 	|| e.contains("quote exceeded")
-		|| e.contains("over quota")
-		// 550 user has too many messages on the server
-		|| e.contains("too many messages")
+	|| e.contains("over quota")
+	// 550 user has too many messages on the server
+	|| e.contains("too many messages")
 }
 
 /// Check if the email account has been disabled or blocked by the email