diff --git a/email_handler.py b/email_handler.py
index 65991111..604ea7c1 100644
--- a/email_handler.py
+++ b/email_handler.py
@@ -406,9 +406,9 @@ def add_alias_to_header_if_needed(msg, alias):
     LOG.d(f"add {alias} to To: header {to_header}")
 
     if to_header:
-        msg[headers.TO] = f"{to_header},{alias.email}"
+        add_or_replace_header(msg, headers.TO, f"{to_header},{alias.email}")
     else:
-        msg[headers.TO] = alias.email
+        add_or_replace_header(msg, headers.TO, alias.email)
 
 
 def replace_header_when_reply(msg: Message, alias: Alias, header: str):
diff --git a/tests/test_email_handler.py b/tests/test_email_handler.py
index 8b71b8d0..53cee299 100644
--- a/tests/test_email_handler.py
+++ b/tests/test_email_handler.py
@@ -191,11 +191,40 @@ def test_dmarc_reply_quarantine(flask_client, dmarc_result):
 
 def test_add_alias_to_header_if_needed():
     msg = EmailMessage()
-    create_new_user()
-    alias = Alias.first()
+    user = create_new_user()
+    alias = Alias.filter_by(user_id=user.id).first()
 
     assert msg[headers.TO] is None
 
     email_handler.add_alias_to_header_if_needed(msg, alias)
 
     assert msg[headers.TO] == alias.email
+
+
+def test_append_alias_to_header_if_needed_existing_to():
+    msg = EmailMessage()
+    original_to = "noone@nowhere.no"
+    msg[headers.TO] = original_to
+    user = create_new_user()
+    alias = Alias.filter_by(user_id=user.id).first()
+    email_handler.add_alias_to_header_if_needed(msg, alias)
+    assert msg[headers.TO] == f"{original_to}, {alias.email}"
+
+
+def test_avoid_add_to_header_already_present():
+    msg = EmailMessage()
+    user = create_new_user()
+    alias = Alias.filter_by(user_id=user.id).first()
+    msg[headers.TO] = alias.email
+    email_handler.add_alias_to_header_if_needed(msg, alias)
+    assert msg[headers.TO] == alias.email
+
+
+def test_avoid_add_to_header_already_present_in_cc():
+    msg = EmailMessage()
+    create_new_user()
+    alias = Alias.first()
+    msg[headers.CC] = alias.email
+    email_handler.add_alias_to_header_if_needed(msg, alias)
+    assert msg[headers.TO] is None
+    assert msg[headers.CC] == alias.email