sgmansell:

Yes, that's what it means. As soonix states, the message after the colon should give you (or your EMail administrator) some further information on it. Because of the spam problem, many places lock out port 25, require authentication and/or alternate protocols. You'll definitely want to talk with your mail server IT team to help figure out what you need to do.

...roboticus

When your only tool is a hammer, all problems look like your thumb.