diff --git a/app/email_utils.py b/app/email_utils.py
index b8769957..0f25353c 100644
--- a/app/email_utils.py
+++ b/app/email_utils.py
@@ -1466,6 +1466,7 @@ def get_verp_info_from_email(email: str) -> Optional[Tuple[VerpType, int]]:
 
 
 def sl_formataddr(name_address_tuple: Tuple[str, str]):
-    """Same as formataddr but use utf-8 encoding by default"""
+    """Same as formataddr but use utf-8 encoding by default and always return str (and never Header)"""
     name, addr = name_address_tuple
-    return formataddr((name, Header(addr, "utf-8")))
+    # formataddr can return Header, make sure to convert to str
+    return str(formataddr((name, Header(addr, "utf-8"))))
diff --git a/tests/test_email_utils.py b/tests/test_email_utils.py
index 7ddf8360..c6e4b216 100644
--- a/tests/test_email_utils.py
+++ b/tests/test_email_utils.py
@@ -782,6 +782,10 @@ def test_add_header_multipart_with_invalid_part():
 
 
 def test_sl_formataddr():
+    # when the name part (first element in the tuple) is empty, formataddr() returns a Header
+    # this makes sure sl_formataddr always returns str
+    assert sl_formataddr(("", "a@b.c")) == "a@b.c"
+
     assert sl_formataddr(("é", "è@ç.à")) == "=?utf-8?b?w6k=?= <è@ç.à>"
     # test that the same name-address can't be handled by the built-in formataddr
     with pytest.raises(UnicodeEncodeError):