diff --git a/apps/smtp/smtp.c b/apps/smtp/smtp.c
index 6a9f828..0d6d483 100644
--- a/apps/smtp/smtp.c
+++ b/apps/smtp/smtp.c
@@ -34,6 +34,7 @@
 #include "lwip/dns.h"
 
 #include <string.h>
+#include <stdlib.h>
 
 /** This is simple SMTP client for raw API.
  * It is a minimal implementation of SMTP as specified in RFC 5321.
@@ -283,16 +284,15 @@ static char smtp_auth_plain[SMTP_MAX_USERNAME_LEN + SMTP_MAX_PASS_LEN + 3];
 /** Length of smtp_auth_plain string (cannot use strlen since it includes \0) */
 static size_t smtp_auth_plain_len;
 
-static size_t max_tx_buf_len;
-static size_t max_rx_buf_len;
-
 static err_t  smtp_verify(const char *data, size_t data_len, u8_t linebreaks_allowed);
 static err_t  smtp_tcp_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t err);
 static void   smtp_tcp_err(void *arg, err_t err);
 static err_t  smtp_tcp_poll(void *arg, struct tcp_pcb *pcb);
 static err_t  smtp_tcp_sent(void *arg, struct tcp_pcb *pcb, u16_t len);
 static err_t  smtp_tcp_connected(void *arg, struct tcp_pcb *pcb, err_t err);
+#if LWIP_DNS
 static void   smtp_dns_found(const char* hostname, ip_addr_t *ipaddr, void *arg);
+#endif /* LWIP_DNS */
 static size_t smtp_base64_encode(char* target, size_t target_len, const char* source, size_t source_len);
 static enum   smtp_session_state smtp_prepare_mail(struct smtp_session *s, u16_t *tx_buf_len);
 static void   smtp_send_body(struct smtp_session *s, struct tcp_pcb *pcb);
@@ -758,6 +758,7 @@ smtp_tcp_connected(void *arg, struct tcp_pcb *pcb, err_t err)
   return ERR_OK;
 }
 
+#if LWIP_DNS
 /** DNS callback
  * If ipaddr is non-NULL, resolving succeeded, otherwise it failed.
  */
@@ -786,6 +787,7 @@ smtp_dns_found(const char* hostname, ip_addr_t *ipaddr, void *arg)
   }
   smtp_close(pcb->callback_arg, pcb, result, 0, err);
 }
+#endif /* LWIP_DNS */
 
 #if SMTP_SUPPORT_AUTH_AUTH || SMTP_SUPPORT_AUTH_LOGIN