K2 Five: MailEvent: The specified string is not in the form required for an e-mail address

I have an input parameter to a K2 Five workflow containing multiple e-mail addresses concatenated with a semi-colon. I can't use this field to send the e-mail to the required addresses, I get the error message 'The specified string is not in the form required for an e-mail address'. I have split the string and tried to use the individual addresses to send the mail with the same result. How do I send the email?