diff --git a/src/apps/smtp/smtp.c b/src/apps/smtp/smtp.c
index 22fa02b6..219af2a0 100644
--- a/src/apps/smtp/smtp.c
+++ b/src/apps/smtp/smtp.c
@@ -194,7 +194,7 @@ enum smtp_session_state {
 
 #ifdef LWIP_DEBUG
 /** State-to-string table for debugging */
-const char *smtp_state_str[] = {
+static const char *smtp_state_str[] = {
   "SMTP_NULL",
   "SMTP_HELO",
   "SMTP_AUTH_PLAIN",
@@ -209,7 +209,7 @@ const char *smtp_state_str[] = {
   "SMTP_CLOSED",
 };
 
-const char *smtp_result_strs[] = {
+static const char *smtp_result_strs[] = {
   "SMTP_RESULT_OK",
   "SMTP_RESULT_ERR_UNKNOWN",
   "SMTP_RESULT_ERR_CONNECT",
@@ -881,7 +881,7 @@ smtp_dns_found(const char* hostname, const ip_addr_t *ipaddr, void *arg)
 #if SMTP_SUPPORT_AUTH_PLAIN || SMTP_SUPPORT_AUTH_LOGIN
 
 /** Table 6-bit-index-to-ASCII used for base64-encoding */
-const u8_t base64_table[] = {
+static const u8_t base64_table[] = {
   'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
   'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
   'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',