This is some stupid misguided privacy rule in Mendix. Normally when you sent a message to the person through his profile page your email was visible for this person. But some misguided person from Mendix thought that this violated your privacy even though you explicitly enter your mail address so the person can contact you.
The only way to reply is to find the persons profile page and use the same method where you do not forget to also enter your own mail address inside the message and not only in the field on top.
Already filled a support ticket for this a long time ago but they still not have fixed this.