Sorted out the problem already - I forgot to set the authentication method. Silly me.
In any case what I did was insert the following line into functions_send_email.asp:
With objCDOSYSCon
'Out going SMTP server
.Fields("http://schemas.microsoft.com/cdo/configuration/smtpserver") = strOutgoingMailServer
'SMTP port
.Fields("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
'CDO Port
.Fields("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
'Timeout
.Fields("http://schemas.microsoft.com/cdo/configuration/smtpconnectiontimeout") = 60
'Login Name
.Fields("http://schemas.microsoft.com/cdo/configuration/sendusername") = "<my username>"
'Password
.Fields("http://schemas.microsoft.com/cdo/configuration/sendpassword" ) = "<my password>"
'Authentication Method
.Fields("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate") = 1
.Fields.Update
End With
Thanks for all the help.