Unable to send Email Notifications behind Corporate Proxy

## Expected Behavior
In a different environment where no proxy server is involved in between Graylog server and the Internet; I am having no issue pushing the email notifications to smtp servers.
However, in my production environment where my server is sitting behind corporate proxy; I am unable to send email notificaitons. Server is properly configured to use proxy server in yum.conf, profile and docker environment.
I am having no issue pulling docker images, yum updates or cloning any repo.

## my docker-compose.yaml
version: '2'
services:
mongodb:
image: mongo:3
volumes:
- mongo_data:/data/db
elasticsearch:
image: docker.elastic.co/elasticsearch/elasticsearch-oss:6.8.10
volumes:
- es_data:/usr/share/elasticsearch/data
environment:
- http.host=0.0.0.0
- transport.host=localhost
- network.host=0.0.0.0
- "ES_JAVA_OPTS=-Xms512m -Xmx512m"
graylog:
image: graylog/graylog:3.3
volumes:
- graylog_data:/usr/share/graylog/data
environment:
- GRAYLOG_PASSWORD_SECRET=xxxxxxxxxxxxxxxxxxxxxxxxxx
- GRAYLOG_ROOT_PASSWORD_SHA2=xxxxxxxxxxxxxxxxxxxxxx
- GRAYLOG_HTTP_EXTERNAL_URI=http://x.x.x.x:9000/
- GRAYLOG_TRANSPORT_EMAIL_ENABLED=true
- GRAYLOG_TRANSPORT_EMAIL_HOSTNAME=smtp.gmail.com
- GRAYLOG_TRANSPORT_EMAIL_PORT=587
- GRAYLOG_TRANSPORT_EMAIL_USE_AUTH=true
- GRAYLOG_TRANSPORT_EMAIL_USE_TLS=true
- GRAYLOG_TRANSPORT_EMAIL_AUTH_USERNAME=xxxxxxx@gmail.com
- GRAYLOG_TRANSPORT_EMAIL_AUTH_PASSWORD=xxxxxxxxxxx
links:
- mongodb:mongo
- elasticsearch
depends_on:
- mongodb
- elasticsearch
ports:
- 9000:9000
- 5140:5140
- 5140:5140/udp
- 12201:12201/udp
volumes:
mongo_data:
driver: local
es_data:
driver: local
graylog_data:
driver: local

## Possible Solution
Do I have to set proxy environment variables in docker-compose as well?

## Steps to Reproduce (for bugs)
1. Use docker compose to pull relevant images.
2. Run the same behind proxy server i.e. pfsense.

## Your Environment
1. CentOS 7 minimal with fairly new version of docker and docker compose.
2. Mongo 3, Graylog 3.3 and ElasticSearch 6.8