Are you in Mendix cloud V3? The documentation says that Mendix mail server only exists in V3.
I usually use a gmail address when I don't have a smtp server. Here is a link to the gmail smtp settings.
Also, if you go this route, sometimes a security alert is sent to the email address you are using to send emails. You have to log in and click the link to authorize the application to use the email address before you can send any emails. \
Hope this helps!