Which version of the Mendix cloud & license do you use? port 25 is only available in v3.
See the documentation; https://docs.mendix.com/deployment/mendixcloud/sending-email
I have a generic gmail account that I use to send emails from a mendix app. I only use this for testing purposes or Proof of Concepts.
Here's a link to the gmail smtp settings.
To whom are you trying to send the email?
I recommend first get it working with Papercut - https://papercut.codeplex.com/
This nice tool allows you to have a mock email server endpoint. It connects to your local machine and listens for emails, and shows them in a similar way that a user would see them in an email client.
If you are in the V4 cloud you can not use the Mendix mailserver anymore (bummer) . You need to set up your own mail server.
When you want to test your email I would suggest SMTP4Dev.
Then you can test locally all the mail functionality.
I use https://sendgrid.com or Mailgun.com for sending email from a Mendix app or Amazon SES.