David,
Thank you very much for the suggestion. I am looking into it.

>Dmitry,
>
>I use http://www.networksorcery.com/enp/protocol/smtp.htm as a reference for SMTP error codes, which describes code 550 as, "Requested action not taken: mailbox unavailable." which means that mailbox 3567 either doesn't exist or is being blocked from receiving email from your SMTP server.
>
>I wrote a program that sent email and I had a problem that was a result of the security and anti-spam settings on the SMTP server that I was using. I sent a lot of identical emails to the same address in a short while and a spam filter decided that I was a spammer. I would make sure that the mailbox exists and then check (or have a network admin check) the log for the SMTP server and see what it has to say about your reject mailing attempts.
>
>David
>
>>The customer gets the following error when emailing from my ASP.NET application:
>>
>>

>>The server rejected one or more recipient addresses.
>>The server response was: 550 5.7.1 Unable to relay for
>>3567@amcomdb1.hospitalname.org
>>

>>
>>I use class System.Web.Mail.SmtpMail for sending emails. I set Exchange Server IP as property .SmptServer. And this setting works when sending regular emails from this application. But when the application attempts to send email to the paging company email (example above, 3567@amcomdb1.hospitalname.org) we get the Unable to Relay error.
>>
>>Can anybody point me to where I should look for the solution to this problem.
>>
>>Thank you.