SMTP 5.7.3 Requested action aborted; user not authenticated. [SOLVED!!]

An e-commerce site that I maintain had been working fine, then all of a sudden emails stopped sending. We started seeing this error: Mailbox unavailable. The server response was: 5.7.3 Requested action aborted; user not authenticated   We hadn’t changed any code, and our username/password worked fine on the web. It turns out that our […]