The specified string is not in the form required for an e-mail address.

I have problem on sending multiple email. MailMessage.To part causes following problems respectively. All rest part of the code is ok. Thanks for any help.

Dim ToAddress As String = "abc1@xxx.com; abc2@xxx.com" / Causes this error: The specified string is not in the form required for an e-mail address.
Dim ToAddress As String = "<abc1@xxx.com>; <abc2@xxx.com>" / Result is: abc1@xxx.com successfull. abc2@xxx.com ignored.
web config:
<system.net>
    <mailSettings>
      <smtp>
        <network
             host="mail.xxx.com"
             port="25"
             userNam ...