Объясните в чем проблема?

Здравствуйте!

Последнее время на один из почтовых ящиков стали сыпаться письма с таким заголовком: Warning: message 1RXT7d-00018q-LO delayed 24 hours и таким содержанием:

This message was created automatically by mail delivery software.

A message that you sent has not yet been delivered to one or more of its

recipients after more than 24 hours on the queue on хххххх.ru.

The message identifier is: 1RXT7d-00018q-LO

The subject of the message is:

=?windows-1251?B?z/Do7uHw5fLl7CDi4PjzIPHy4PDz/iDs5eHl6/w=?=

The date of the message is: Mon, 5 Dec 2011 10:37:47 +0300

The address to which the message has not yet been delivered is:

rexx00@boom.ru - адреса разные постоянно

No action is required on your part. Delivery attempts will continue for

some time, and this warning may be repeated at intervals if the message

remains undelivered. Eventually the mail delivery software will give up,

and when that happens, the message will be returned to you.

Заранее благодарен!