550 Unknown User Error

Hi, we have a registration  form and it runs smoothly until recently there is an error message:

<!-- Web.Config Configuration File -->
<Configuration>
 <system.web>
  <customErrors mode ="Off"/>
   </system.web>
</configuration> 

 Then I tested from the dedicated server and got the following errror:

The server rejected one or more recipinet address. The server response was: 550 Unknown user

Line 133 smtpMail.Send(objMM)


It looks like when user registered with fake email address, the system will give above error message, otherwise everything will be fine.

Is there any way to give a message(the email address is not valid) to the user when he(she type in a wrong email address) instead of giving the error message above? Appreciate it!
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