Exception occurred while flushing email queue: Connection could not be established with host smtp.gmail.com [Network is unreachable #101]

Как использовать smtp от Gmail

app/config/parameters.yml

mailer_transport: smtp
mailer_host: smtp.gmail.com
mailer_user: <твой емейл gmail>
mailer_password: <пароль от ящика>
mailer_auth_mode: null
mailer_port: 587
mailer_encryption: tls

app/config/config.yml

swiftmailer:
    transport: "%mailer_transport%"
    host:      "%mailer_host%"
    username:  "%mailer_user%"
    password:  "%mailer_password%"
    spool:     { type: memory }
    auth_mode: '%mailer_auth_mode%'
    port:      '%mailer_port%'
    encryption: '%mailer_encryption%'