"I just tried to create a new user and got that popup or something like it.."
Who, What, Where, When, are important.. why often adds context if it's not obvious..
You got "who" ( "I just tried.."), and "what" ("create a new user"), and ""when" (apparently during the process of creating a new user), but where (You asking if it was "us folks" implied this website, but no..). If you had specified where you were trying to create the new user (your own server) would have elicited a more accurate response - I recognized the mail() routine error as a server response, just didn't know what server you were referring to.
Your subsequent question regarding SMTP "client" is also scarce on specifics that helps cut to the chase, and leaves us trying to imagine what exactly you're trying to do that generated your question. I answered the question you asked, but I'm not sure it addresses the problem that gave rise to the question in the first place.. context is really helpful..