Why won't their server accept my e-mail??

[HANDzCLEANx]

Hi

A while ago(7 months) I created a web-site for my mother/her business.

Since I was the creator, I was also the webmaster.
So as my privelage for being webmaster I, ofcourse, to the webmaster's email address( webmaster@site.com)

So , my prob is why wont some(only a few) servers accept my e-mail.
The last message I had was this( I removed all my site details):

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'webmaster@google.com'. Subject 'Glitch in the system?', Account: 'pop3.mysite.co.za', Server: 'smtp.mysite.co.za', Protocol: SMTP, Server Response: '553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1)', Port: 25, Secure(SSL): No, Server Error: 553, Error Number: 0x800CCC79

Their(server) reason( as stated above) :

'553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1)'

.

Might it even be that I dont have a "secure" server:

Secure(SSL): No,

I have a not-so urgent message that for Google's webmaster.
I dont remember seeing this specific error before.
Any help would be great.
(As I read through my post, it hit me that this thread could fit better in another forum.
Sorry in advance)

Cheers

[MrLinus]

My ISP has a special blocker to prevent emails from being received from "relayed" SMTP servers. It's a method of dealing with some of the spammers and works effectively. Try using something like a web-based mail to send it out. When you do, you can then inform them of the problem and see if they can remove the blocker from your address at least.

[HANDzCLEANx]

thanx
I will definitly do that
Thank you again...