CRM 4.0 Email Router error

I have a problem with incoming e-mail. Messages always get to folder undeliverable. And so they don't get to queue in CRM. This happens only with messages from one e-mail address. So I installed Crm 4 Email Connector Logger and get and error for this message:

04/09/2012 16:32:55 : #20669 - Checking for e-mail messages to process in mailbox CRM_Clients_Support@ourcompany.com.
04/09/2012 16:32:55 : #49073 - Validating the e-mail message with subject "Email subject" from "Client Name Surname" <name.surname@ourclient.com>" to CRM Clients Support (CRM_Clients_Support@ourcompany.com) <CRM_Clients_Support@ourcompany.com> for delivery to CRM_Clients_Support@ourcompany.com.
04/09/2012 16:32:55 : #1866 - Checking the e-mail message with subject "Email subject" in mailbox CRM_Clients_Support@ourcompany.com for delivery to http://crm.ourcompany.com/OURCompany.
04/09/2012 16:32:56 : #9466 - Accepting the e-mail message with subject "Email subject" in mailbox CRM_Clients_Support@ourcompany.com for delivery to http://crm.ourcompany.com/OURCompany.
04/09/2012 16:32:56 : #46519 - Delivering the e-mail message with subject "Email subject" in mailbox CRM_Clients_Support@ourcompany.com for delivery to http://crm.ourcompany.com/OURCompany.
04/09/2012 16:32:58 : #9628 - An error occurred while delivering the e-mail message with subject "Email subject" in mailbox CRM_Clients_Support@ourcompany.com for delivery to http://crm.ourcompany.com/OURCompany. System.Web.Services.Protocols.SoapException: Server was unable to process request.
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
   at Microsoft.Crm.Sdk.Proxy.CrmService.Execute(Request Request)
   at Microsoft.Crm.Tools.Email.Providers.CrmPollingMailboxProvider.DeliverMessageInternal(EmailMessage emailMessage)
   at Microsoft.Crm.Tools.Email.Providers.ExchangePollingMailboxProvider.DeliverMessageInternal(EmailMessage emailMessage)
   at Microsoft.Crm.Tools.Email.Providers.CrmPollingMailboxProvider.DeliverMessage(EmailMessage emailMessage)
   at Microsoft.Crm.Tools.Email.Providers.CrmPollingMailboxProvider.Run()\r\n
  0x80044192
  Invalid e-mail address. For more information, contact your system administrator.
  Platform

04/09/2012 16:32:58 : #30463 - Moving the e-mail message with subject ""Client Name Surname" <name.surname@ourclient.com>" email subject to the undeliverable folder because it has been marked as undeliverable.


What can be the problem with this address?