We have two external customers who use Zoho mail servers and neither of them have been able to send e-mail to our organization.  One customer has the domain of shineslp.com and the other aspenpatch.com.  No e-mail sent to recipients in our organization ever arrives.  

I am the system admin and we have a spam filter in front of our Exchange server.  I do not see these messages ever even arriving to our spam filter.  Even if the e-mails are quarantined or deleted by the spam filter, they show up in the log.  These e-mails however do not appear in our logs.

One of the customers forwarded me a copy of the bounceback e-mail she received.  It contains the following:

   ----- The following addresses had fatal errors -----    
  [Status: Error, Address: <my e-mail address>, ResponseCode 421, , null]

Message will be retried for 4 more day(s)

Even the conformation e-mail I should have received for signing up an account on these forums never arrived.  No record of it at all on my spam filter.