error 0x800cc79 in outlook express when trying to reply to emails using optus mobile broadband

Hi I have a client that has recently purchased an optus usb mobile broadband stick. ( os XP Home)
Using outlook express 6 she can send and receive no problems, however when she tries to reply to emails as appose to creating a new message she gets the following message  "The message could not be sent because one of the recipients was rejected by the server"
 Error number: 0x800CC79
The server settings are correct.  The return addresses are valid (she has used them for ages)
She can use the reply function when logged on to web mail.
Has anyone any suggestions?

dig59Asked:
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dig59Author Commented:
The solution was the client had two email accounts both with optus
the secondry was an old dialup mail account the client had paid for and maintaned over the years.
this account was removed via OE tools > accounts > Mail (tab)  > remove
close and reopen outlook.
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