It says that you need a secure (ssl) connection to authenticate. I think they must have changed this recently. I dont know if Zencart support smtpauth with a secure connection. I havent found a setting in the Email options. But i saw this thread Its a bit too long for me now :) Have a look yourself, maybe it can solve your problems.

Have you tried both ports offered by gmail

smtp.gmail.com (use authentication)
Use Authentication: Yes
Use STARTTLS: Yes (some clients call this SSL)
Port: 465 or 587