I tried explaining to my host but they don't seem to understand.

To explain it better to them can you tell me how this form is sent https://mylittleforum.net/forum/index.php?mode=contact&user_id=100799

does it use your main email address?

Like I said if I use the envelope to contact admin the message is delivered but if I use the envelope to contact any other user the message does not get delivered like it is being blocked.