Asp form won't send to other domains

I have this form which works fine, but I wanted to send an email to the person who filled out the form for their record and confirmation that all data is correct.

set imsg = createobject("cdo.message")
    set iconf = createobject("cdo.configuration")
    Set Flds = iConf.Fields
    With Flds
   .Item ("") = 2
   .Item ("") = smtp
     .Item ("") = 25
     .Item ("") = 60
   End With
   With iMsg
   Set .Configuration = iConf
        .To = "[email protected]"       'site visitor
        .Bcc = "[email protected]"      'send to us
        .From = "[email protected]"  
        .Subject = Subject
        .HTMLBody = HTML
    End With
    set imsg = nothing
    set iconf = nothing
    set HTML = nothing    

it sends w/o a problem when the domain1 is used, but not when another domain name is used.

can someone more knowledgeable explain why?