554-Your IP address **.**.**.** does not have a valid PTR record associated with it. You will need to add a PTR record (also known as reverse lookup) before you are able to send email into the some network.

Client host rejected: cannot find your hostname, [**.**.**.**]

Решение: нужно попросить Вашего провадера прописать в своих ДНС обратную записть **.**.**.**.in-addr.arpa.