Unable to connect GitLab with Mailgun

I am unable to send emails in GitLab, I am using the service Mailgun, below my settings:

sudo vim /etc/gitlab/gitlab.rb 

gitlab_rails['gitlab_email_from'] = "username@domain.com"
gitlab_rails['gitlab_email_reply_to'] = "username@domain.com"

gitlab_rails['smtp_enable'] = true
gitlab_rails['smtp_address'] = "smtp.mailgun.org"
gitlab_rails['smtp_port'] = 587
gitlab_rails['smtp_authentication'] = "plain"
gitlab_rails['smtp_enable_starttls_auto'] = true
gitlab_rails['smtp_user_name'] = "username@domain.com"
gitlab_rails['smtp_password'] = "secret"
gitlab_rails['smtp_domain'] = "domain.com"

sudo gitlab-ctl reconfigure
sudo gitlab-ctl restart

Does anyone have any idea how to solve this? Thanks.