Many Servers won’t allow masked domain name like localhost, This has to be replaced with ACTUAL mail server adrress like mail.YOURSERVERNAME.com
This will solve the mail issues and also Error in generating Invoces in SIMPLE INOICES application
Change the mail host in factories.yml in below path
/home/YOUR SERVER.COM/public_html/billing/lib/vendor/symfony/lib/config/config/factories.yml
mailer:
class: sfMailer
param:
logging: %SF_LOGGING_ENABLED%
charset: %SF_CHARSET%
delivery_strategy: realtime
transport:
class: Swift_SmtpTransport
param:
host: mail.YOURSERVERNAME.com
port: 25
encryption: ~
username: ~
password: ~