Postfix doesn't resolve my container hostname

here my problem.

I've created an app which send email with postfix. And i would like to received these emails with maildev with another container.

My docker compose looks like this

mailer:
    depends_on:
      - maildev
    build: ./mailer
    ports: 
      - "5050:5050" 
  maildev:
    image: maildev/maildev
    ports: 
      - "1025:1025"
      - "1080:1080"

and my main.cf has relayhost = maildev:1025

I can ping maildev from mailer but my relayhost doesn't work.

If I put the ip of the maildev container :1025 in the main.cf it works.

But why maildev:1025 isn't resolved ?

How can I resolve the right ip with the port in the main.cf of postfix ?