diff --git a/email_validator/deliverability.py b/email_validator/deliverability.py
index 3cddc77..0b122d7 100644
--- a/email_validator/deliverability.py
+++ b/email_validator/deliverability.py
@@ -8,7 +8,14 @@
 import dns.exception
 
 
+def _reject_bool_timeout(timeout: Optional[int]) -> None:
+    # bool subclasses int; timeout=True would silently become lifetime 1s
+    if isinstance(timeout, bool):
+        raise TypeError("timeout must be an int or float, not bool")
+
+
 def caching_resolver(*, timeout: Optional[int] = None, cache: Any = None, dns_resolver: Optional[dns.resolver.Resolver] = None) -> dns.resolver.Resolver:
+    _reject_bool_timeout(timeout)
     if timeout is None:
         from . import DEFAULT_TIMEOUT
         timeout = DEFAULT_TIMEOUT
@@ -34,6 +41,7 @@ def validate_email_deliverability(domain: str, domain_i18n: str, timeout: Option
     # If no dns.resolver.Resolver was given, get dnspython's default resolver.
     # Override the default resolver's timeout. This may affect other uses of
     # dnspython in this process.
+    _reject_bool_timeout(timeout)
     if dns_resolver is None:
         from . import DEFAULT_TIMEOUT
         if timeout is None:
diff --git a/email_validator/validate_email.py b/email_validator/validate_email.py
index 07b398a..9cc46c7 100644
--- a/email_validator/validate_email.py
+++ b/email_validator/validate_email.py
@@ -58,6 +58,9 @@ def validate_email(
         globally_deliverable = GLOBALLY_DELIVERABLE
     if timeout is None and dns_resolver is None:
         timeout = DEFAULT_TIMEOUT
+    # bool subclasses int; timeout=True would silently become DNS lifetime 1s
+    if isinstance(timeout, bool):
+        raise TypeError("timeout must be an int or float, not bool")
 
     if isinstance(email, str):
         pass
diff --git a/tests/test_deliverability.py b/tests/test_deliverability.py
index e1307c2..9505cb4 100644
--- a/tests/test_deliverability.py
+++ b/tests/test_deliverability.py
@@ -90,3 +90,18 @@ def put(self, key: Any, value: Any) -> Any:
 
     validate_email("test@gmail.com", dns_resolver=resolver)
     assert len(cache.cache) == 1
+
+
+def test_timeout_rejects_bool() -> None:
+    """bool subclasses int; timeout=True must not silently become lifetime 1s."""
+    import pytest
+    from email_validator.deliverability import caching_resolver, validate_email_deliverability
+
+    for value in (True, False):
+        with pytest.raises(TypeError, match="timeout must be an int or float, not bool"):
+            caching_resolver(timeout=value)
+        with pytest.raises(TypeError, match="timeout must be an int or float, not bool"):
+            validate_email_deliverability("example.com", "example.com", timeout=value)
+    # valid int still accepted on caching_resolver
+    resolver = caching_resolver(timeout=5)
+    assert resolver.lifetime == 5