Hi Peno, I had this problem when I switched broadband provider - I was using eircom dial-up then went with another providers DSL. The outgoing smtp server wouldn't allow me to send mail that originated from another host, i.e. (I believe!) the eircom mail server would not allow me to send mail through it as it had originated on my new DSl provider. The only mail I could send was to an eircom address.
The way I got around this was to set up a yahoo mail (or a gmail) account. They both allow mail to be sent through their mail servers regardless of the network you are using.
HTH,
E.
PS the error is coming from the mail server and not mcafee, as Blinder has said. Mcafee sits between your mail program and the mail server so it can scan incoming and outgoing mail for viruses etc. It gives you any error responses it gets from the mail server. The 553 error message you have is, like many of the internet error messages unhelpful and misleading!