Are you a Charter.net customer and connected to the Internet via them
when trying to send?

See:
E-Mail Client Error Messages
http://charter.com/customers/support.aspx?supportarticleid=1416

For how to configure your e-mail program, see:
http://charter.com/customers/support.aspx?menuitem=10#

Are you sure that the e-mail address you are sending to
dusty009@charter.net is valid?

And although you posted in an OE 5.5 newsgroup, the error message
suggests that you are using Vista Window Mail.

Windows Vista Mail Newsgroup:
news://msnews.microsoft.com/microsoft.public.windows.vista.mail

You can also get to the newsgroup via this web site:
http://www.microsoft.com/communities/newsgroups/default.mspx
There select Windows Vista, then Windows Mail.

550 Relaying errors:
http://www.insideoe.com/problems/errors.htm#550relay
550, 553: http://office.microsoft.com/en-us/assistance/HA011128331033.aspx

I am right there with you.  For the last 5 years or more, I have been a
Charter Customer, and my outgoing email has gone out fine with
smtp.charter.net port 25.  All of a sudden, about 2 months ago, it started
working intermittently, and now it doesn't seem to work at all.  Their tech
support was of no help... they said that I am connected to Bellsouth (even
though my phone, cable, and internet are through Charter and I have nothing
through Bellsouth), and that they weren't responsible for giving me my
outgoing connection strings.

I don't know what to do to get my outgoing mail set up and working again.  I
tried a guess at using "smtp.bellsouth.net" which actually worked
intermittently for a bit, but again doesn't work.

If you have any solutions, please help.  I have tried the settings with
Entourage (Outlook for the MAC), and with Apple Mail, with the same
connection issue on outgoing mail.  The incoming mail with POP.Charter.net
still works just perfectly.

Thanks,

Annie