diff --git a/include/h.h b/include/h.h
index fa3569b0a..6b380f5a4 100644
--- a/include/h.h
+++ b/include/h.h
@@ -1158,6 +1158,7 @@ extern LogData *log_data_socket_error(int fd);
 extern LogData *log_data_link_block(ConfigItem_link *link);
 extern LogData *log_data_tkl(const char *key, TKL *tkl);
 extern LogData *log_data_tls_error(void);
+extern void log_pre_rehash(void);
 extern int log_tests(void);
 extern void config_pre_run_log(void);
 extern void log_blocks_switchover(void);
@@ -1170,6 +1171,7 @@ extern int valid_subsystem(const char *s);
 extern const char *timestamp_iso8601_now(void);
 extern const char *timestamp_iso8601(time_t v);
 extern int is_valid_snomask(char c);
+extern int is_valid_snomask_string_testing(const char *str, char **wrong);
 /* end of logging */
 extern void add_fake_lag(Client *client, long msec);
 extern char *prefix_with_extban(const char *remainder, BanContext *b, Extban *extban, char *buf, size_t buflen);
diff --git a/src/conf.c b/src/conf.c
index abf8068ab..f3d78dff1 100644
--- a/src/conf.c
+++ b/src/conf.c
@@ -246,6 +246,7 @@ MODVAR int			config_error_flag = 0;
 int			config_verbose = 0;
 
 int need_operclass_permissions_upgrade = 0;
+int invalid_snomasks_encountered = 0;
 int have_tls_listeners = 0;
 char *port_6667_ip = NULL;
 
@@ -1893,11 +1894,6 @@ void applymeblock(void)
 		strlcpy(me.id, conf_me->sid, sizeof(me.id));
 }
 
-/** Reset config tests (before running the config test) */
-void config_test_reset(void)
-{
-}
-
 /** Run config test and all post config tests. */
 int config_test_all(void)
 {
@@ -2049,6 +2045,7 @@ int config_test(void)
 	memset(&nicklengths, 0, sizeof(nicklengths));
 	config_setdefaultsettings(&tempiConf);
 	clicap_pre_rehash();
+	log_pre_rehash();
 	free_config_defines();
 
 	if (!config_loadmodules())
@@ -2059,8 +2056,6 @@ int config_test(void)
 
 	preprocessor_resolve_conditionals_all(PREPROCESSOR_PHASE_MODULE);
 
-	config_test_reset();
-
 	if (!config_test_all())
 	{
 		config_error("IRCd configuration failed to pass testing");
@@ -2747,6 +2742,21 @@ int	config_test_blocks()
 	int		errors = 0;
 	Hook *h;
 
+	invalid_snomasks_encountered = 0;
+
+	/* First, all the log { } blocks everywhere */
+	for (cfptr = conf; cfptr; cfptr = cfptr->next)
+	{
+		if (config_verbose > 1)
+			config_status("Testing %s", cfptr->filename);
+		/* First test and run the log { } blocks */
+		for (ce = cfptr->items; ce; ce = ce->next)
+		{
+			if (!strcmp(ce->name, "log"))
+				errors += config_test_log(cfptr, ce);
+		}
+	}
+
 	for (cfptr = conf; cfptr; cfptr = cfptr->next)
 	{
 		if (config_verbose > 1)
@@ -2773,7 +2783,8 @@ int	config_test_blocks()
 		{
 			/* These are already processed, so skip them here.. */
 			if (!strcmp(ce->name, "secret") ||
-			    !strcmp(ce->name, "set"))
+			    !strcmp(ce->name, "set") ||
+			    !strcmp(ce->name, "log"))
 			{
 				continue;
 			}
@@ -2836,6 +2847,12 @@ int	config_test_blocks()
 		config_error("%i errors encountered", errors);
 	}
 
+	if (invalid_snomasks_encountered)
+	{
+		config_error("It seems your set::snomask-on-oper and/or oper::snomask needs to be updated. Are you perhaps upgrading from an older version to UnrealIRCd 6?");
+		config_error("See https://www.unrealircd.org/docs/Upgrading_from_5.x#Update_your_snomasks");
+	}
+
 	return (errors > 0 ? -1 : 1);
 }
 
@@ -4033,12 +4050,20 @@ int	_test_oper(ConfigFile *conf, ConfigEntry *ce)
 			/* oper::snomask */
 			else if (!strcmp(cep->name, "snomask"))
 			{
+				char *wrong_snomask;
 				if (has_snomask)
 				{
 					config_warn_duplicate(cep->file->filename,
 						cep->line_number, "oper::snomask");
 					continue;
 				}
+				if (!is_valid_snomask_string_testing(cep->value, &wrong_snomask))
+				{
+					config_error("%s:%i: oper::snomask contains unknown snomask letter(s) '%s'",
+					             cep->file->filename, cep->line_number, wrong_snomask);
+					errors++;
+					invalid_snomasks_encountered++;
+				}
 				has_snomask = 1;
 			}
 			else if (!strcmp(cep->name, "server-notice-colors"))
@@ -7808,8 +7833,16 @@ int	_test_set(ConfigFile *conf, ConfigEntry *ce)
 			set_usermode(cep->value);
 		}
 		else if (!strcmp(cep->name, "snomask-on-oper")) {
+			char *wrong_snomask;
 			CheckNull(cep);
 			CheckDuplicate(cep, snomask_on_oper, "snomask-on-oper");
+			if (!is_valid_snomask_string_testing(cep->value, &wrong_snomask))
+			{
+				config_error("%s:%i: set::snomask-on-oper contains unknown snomask letter(s) '%s'",
+					     cep->file->filename, cep->line_number, wrong_snomask);
+				errors++;
+				invalid_snomasks_encountered++;
+			}
 		}
 		else if (!strcmp(cep->name, "server-notice-colors")) {
 			CheckNull(cep);
diff --git a/src/log.c b/src/log.c
index c3585684b..d21dabbca 100644
--- a/src/log.c
+++ b/src/log.c
@@ -36,6 +36,7 @@ Log *temp_logs[NUM_LOG_DESTINATIONS] = { NULL, NULL, NULL, NULL, NULL };
 static int snomask_num_destinations = 0;
 
 static char snomasks_in_use[257] = { '\0' };
+static char snomasks_in_use_testing[257] = { '\0' };
 
 /* Forward declarations */
 int log_sources_match(LogSource *logsource, LogLevel loglevel, const char *subsystem, const char *event_id, int matched_already);
@@ -205,6 +206,8 @@ int config_test_log(ConfigFile *conf, ConfigEntry *block)
 						config_error("%s:%d: snomask must be a single letter",
 							cep->file->filename, cep->line_number);
 						errors++;
+					} else {
+						strlcat(snomasks_in_use_testing, cep->value, sizeof(snomasks_in_use_testing));
 					}
 				} else
 				if (!strcmp(cep->name, "channel"))
@@ -1803,6 +1806,12 @@ void postconf_defaults_log_block(void)
 	AppendListItem(ls, l->sources);
 }
 
+/* Called before CONFIG_TEST */
+void log_pre_rehash(void)
+{
+	*snomasks_in_use_testing = '\0';
+}
+
 /* Called after CONFIG_TEST right before CONFIG_RUN */
 void config_pre_run_log(void)
 {
@@ -1828,3 +1837,35 @@ int is_valid_snomask(char c)
 {
 	return strchr(snomasks_in_use, c) ? 1 : 0;
 }
+
+/** Check if a letter is a valid snomask during or after CONFIG_TEST
+ * (the snomasks exist in the log block configuration read during config_test).
+ * @param c	the snomask letter to check
+ * @returns	1 if exists, 0 if not.
+ */
+int is_valid_snomask_testing(char c)
+{
+	return strchr(snomasks_in_use_testing, c) ? 1 : 0;
+}
+
+/** Check if a string all consists of valid snomasks during or after CONFIG_TEST
+ * (the snomasks exist in the log block configuration read during config_test).
+ * @param str			the snomask string to check
+ * @param invalid_snomasks	list of unknown snomask letters
+ * @returns			1 if exists, 0 if not.
+ */
+int is_valid_snomask_string_testing(const char *str, char **invalid_snomasks)
+{
+	static char invalid_snomasks_buf[256];
+
+	*invalid_snomasks_buf = '\0';
+	for (; *str; str++)
+	{
+		if ((*str == '+') || (*str == '-'))
+			continue;
+		if (!strchr(snomasks_in_use_testing, *str))
+			strlcat_letter(invalid_snomasks_buf, *str, sizeof(invalid_snomasks_buf));
+	}
+	*invalid_snomasks = invalid_snomasks_buf;
+	return *invalid_snomasks_buf ? 0 : 1;
+}