multiple addressees for sendmail?

multiple addressees for sendmail?

Before upgrade to Zoho 3.0, my script was working fine to sendmail to multiple individuals selected in a form.  I concatenated their email addresses together to form the "To:" using the format:

 email1@here.com, email2@here.com, email3@here.com

Upon upgrade this feature broke and I am just now trying to debug.  I've tried  commas with no spaces, semi-colons with spaces and no spaces, making sure there is no trailing punctuation... everything I can think of. 

I can have concatenated email adresses with semicolons, no spaces as a valid email address on a form and sendmail will work, but I cant seem to do the same via script.

What is the proper format to string together multiple email addresses for sendmail?

I could rewrite the whole script to iterate through each address one at a time, but I actually want all recipients to see who else received the email and to be able to reply-all in response.

Thanks.