diff --git a/compose.mailrelay.yml b/compose.mailrelay.yml
index 89f0779..1de0783 100644
--- a/compose.mailrelay.yml
+++ b/compose.mailrelay.yml
@@ -8,7 +8,7 @@ services:
       - smtp_password
     environment:
       - SMTP_HOST=${SMTP_HOST}
-      - SMTP_PORT=${SMTP_PORT}
+      - SMTP_PORT=${SMTP_PORT:25}
       - SMTP_AUTH=${SMTP_AUTH}
       - SMTP_TLS=${SMTP_TLS}
       - MAIL_FROM=${MAIL_FROM}