Sending email from dockerized web server

I have a web server in a docker container and I want to send emails from it to outside world using mailtrap.io smtp server. I tried to send the email, but received an error:

 _sendEmail couldn't send email: messages@example.com Error: exception 'SocketException' with message
'Cannot assign requested address' in /var/www/html/core/lib/Cake/Network/CakeSocket.php:0 Stack trace: #0
/var/www/html/core/lib/Cake/Network/Email/SMTPTransport.php(0): CakeSocket-connect() #1
/var/www/html/core/lib/Cake/Network/Email/SMTPTransport.php(0): SmtpTransport-_connect() #2
/var/www/html/core/lib/Cake/Network/Email/CakeEmail.php(0): SmtpTransport-send(Object(CakeEmail)) #3
/var/www/html/app/Model/AppModel.php(470): CakeEmail->send(Array) #4 

How do I send email outside of an existing docker container?