thank you for your reply and I apologize for this passed time without my reply.
There was an evolution in this time. In fact I have created a subscription form and I have the same problem. It seems that the only e-mail addresses that are in the same domain (domainname.com and firstname.lastname@example.org, ....@domainname.com) receive the form.
Initially I received the form too with another domain, but meanwhile my mail server is changed and from then on I don't receive e-mail from form more.
I ask the support of the server administrators, because I suppose that my previous mail server was in the same server of the website and his form.
Have you another ideas?