You SHOULD use above configuration to send / receive email for @rpxholding.com no matter what ISP you used to connect to Internet.
if you are not using above configuration, below errors will came out when you are trying to send email
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The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'c.guifu@gmail.com'. Subject 'Re: Package: 8599 7443 7454', Account: '202.155.64.99', Server: 'smtp.rpxholding.com', Protocol: SMTP, Server Response: '553 sorry, that domain isn't in my list of allowed rcpthosts (#5.5.3 - chkuser)', Port: 25, Secure(SSL): No, Server Error: 553, Error Number: 0x800CCC79
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Your message did not reach some or all of the intended recipients.
Subject: RE: Wedding Invitation -Agus&Melly-
Sent: 03/06/2008 17:45
The following recipient(s) could not be reached:
'mellyana yang' on 03/06/2008 17:45
553 sorry, that domain isn't in my list of allowed rcpthosts (#5.5.3 - chkuser)
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Your message did not reach some or all of the intended recipients.
Subject: yahoo
Sent: 01/07/2010 15:09
The following recipient(s) cannot be reached:
'am3n_group@yahoo.com' on 01/07/2010 15:09
553 Requested action not taken: mailbox name not allowed
