diff --git a/changelog.d/6379.misc b/changelog.d/6379.misc
new file mode 100644
index 000000000000..725c2e7d8744
--- /dev/null
+++ b/changelog.d/6379.misc
@@ -0,0 +1 @@
+Complain on startup instead of 500'ing during runtime when `public_baseurl` isn't set when necessary.
\ No newline at end of file
diff --git a/synapse/config/emailconfig.py b/synapse/config/emailconfig.py
index 43fad0bf8b27..ac1724045f04 100644
--- a/synapse/config/emailconfig.py
+++ b/synapse/config/emailconfig.py
@@ -146,6 +146,8 @@ def read_config(self, config, **kwargs):
                 if k not in email_config:
                     missing.append("email." + k)
 
+            # public_baseurl is required to build password reset and validation links that
+            # will be emailed to users
             if config.get("public_baseurl") is None:
                 missing.append("public_baseurl")
 
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index 1f6dac69da00..ee9614c5f716 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -106,6 +106,13 @@ def read_config(self, config, **kwargs):
         account_threepid_delegates = config.get("account_threepid_delegates") or {}
         self.account_threepid_delegate_email = account_threepid_delegates.get("email")
         self.account_threepid_delegate_msisdn = account_threepid_delegates.get("msisdn")
+        if self.account_threepid_delegate_msisdn and not self.public_baseurl:
+            raise ConfigError(
+                "The configuration option `public_baseurl` is required if "
+                "`account_threepid_delegate.msisdn` is set, such that "
+                "clients know where to submit validation tokens to. Please "
+                "configure `public_baseurl`."
+            )
 
         self.default_identity_server = config.get("default_identity_server")
         self.allow_guest_access = config.get("allow_guest_access", False)
diff --git a/tests/rest/client/v2_alpha/test_register.py b/tests/rest/client/v2_alpha/test_register.py
index dab87e5edff8..c0d0d2b44eb9 100644
--- a/tests/rest/client/v2_alpha/test_register.py
+++ b/tests/rest/client/v2_alpha/test_register.py
@@ -203,6 +203,7 @@ def test_advertised_flows(self):
 
     @unittest.override_config(
         {
+            "public_baseurl": "https://test_server",
             "enable_registration_captcha": True,
             "user_consent": {
                 "version": "1",