What email settings are you using? If you are using localhost port 25 without login etc, the mail server listening there is quite permissive about what you submit.
If so, make sure you use a From address on which you can actually receive email. Whenever the outgoing mail server at Mendix encounters an issue (e.g. the remote mail server on the domain of your target email address refuses to accept the message), it will try to send an error report to the address you specified as from address. If sending an email to the from address is not possible, it will appear as if your original message went into some blackhole.
There was a significant delay in receiving the email, but all seems to be working now.