who can explain why I do not receive messages in the mail? ( maybe because of the mail provider?)
and what should I do in such a situation?!(to change the provider?)
If you are talking about email responses to your web forms, it may be that your hosting company does not allow the sending of emails to domains other than your own. If, for example, you are trying to have email messages sent to a gmail or yahoo address, your hosting company may not allow it. Check with the hosting company first to see if they can give you some guidance.
The ideal solution is to set up an email account in your own domain (e.g.info at your domain) - this is something that is normally included with most hosting packages. You would then set up that email as the receiving email address. This will usually resolve the problem.