What kind of mail do you use? If you e.g. use JMail, you will have to change inc_mail.asp as discussed in the topic "MOD: Email Subscription, version 4" (page 2)
If you use JMail, you must split the string strRecipients in inc_mail.asp like this:
Set objNewMail = Server.CreateObject("Jmail.smtpmail")
objNewMail.ServerAddress = strMailServer
arrayStr = Split(strRecipients, ";", -1, vbTextCompare)
For Each emailStr in arrayStr
objNewMail.AddRecipient emailStr
Next
objNewMail.Sender = strSender
objNewMail.Subject = strSubject
objNewMail.body = strMessage