update mail server hostname

docker-mailserver/docker-compose.yml

@@ -8,7 +8,7 @@ services:
     container_name: mailserver
     # If the FQDN for your mail-server is only two labels (eg: example.com),
     # you can assign this entirely to `hostname` and remove `domainname`.
-    hostname: mail.cups-1.aosus.org
+    hostname: cups-1.aosus.org
     env_file: mailserver.env
     # More information about the mail-server ports:
     # https://docker-mailserver.github.io/docker-mailserver/latest/config/security/understanding-the-ports/