docker-compose not passing environment variables for 'external_links' container

I have a container that is specified to docker-compose like so:

postfix:
    build: postfix
    ports:
        - "25:25"
    volumes_from:
        - mailspool
    links:
        - dovecot:dovecot
    external_links:
        - mariadb_single_container:mariadb
    env_file: environment

It was working fine, but after a system reboot, the environment variables for linking to the mariadb container are not being presented to the postfix container.

I've tried stopping and starting the postfix container in case it was something to do with the order things came up in. I'm wary of taking down mariadb, which other containers are also depending on, but mariadb looks normal, and other containers are using it successfully (those containers don't use docker-compose).

Any ideas?