Problems e-mailing someone@yahoo.com.tw from Outlook Express

[#1]

When I use Outlook Express to send an e-mail to someone@yahoo.com.tw in Taiwan, Outlook Express reports the following error message :

An unknown error has occurred. Subject 'BLAH', Account: 'myname@mydomain.com', Server: 'relay.plus.net', Protocol: SMTP, Port: 25, Secure(SSL): No, Error Number: 0x800CC0B. 
I know another PlusNet customer who gets the same error when sending to this same *.tw address from their Outlook Express installation.

The address we have for our friend in Taiwan is valid e-mail address; we have received e-mails from them. If I send a message to this address via the PlusNet WebMail interface, the recipient *does* receive my message.

My Outlook Express configuration settings for sending mail are in line with what is advised on the PlusNet support pages. I can happily e-mail everyone else I know; I only get the above error if I try to to e-mail someone@yahoo.com.tw
What's worse is that if my message to someone@yahoo.com.tw is the first in a queue of several messages to be sent, then when the first message fails, the rest of the messages in the queue never make it out.

Could it be that the PlusNet SMTP server may not recognise or may have black-listed either *.tw or *.com.tw and thus returns an error ? Or does Outlook Express try to be too smart and try to validate someone@yahoo.com.tw ?
Could someone else please try e-mailing someone@yahoo.com.tw from their Outlook Express installation via your SMTP server and let me know if they get the same error message ?
Thanks.

[#2]

Hi Kyoto,
The 0B error usually means that outlook can't connect to the internet, usually due to firewall or antivirus software. Can you send to the address using webmail (http://webmail.plus.net)? We've not heard of anyone else having these issues as yet.

[#3]

Hi there,
I submitted the same question to the support team (ID : 30838624). The response from one of your colleagues on Sat 21 Nov 2009 was :

Quote

Thanks for contacting us regarding this. I can confirm that there is an issue sending to these addresses through the regular relay server but not through the webmail server. As such, I am passing this through internally to the networks team for further investigation. [-=Internal=-] As above, email rejected when trying to send via relay.plus.net but sent without any reported issues when sending via webmail.

It is bizarre : via webmail.plus.net it works fine but via Outlook Express and the SMTP relay server, it returns an error. And it's just for this address to Taiwan - I can happily e-mail everyone else I know via Outlook Express. I wonder if someone@yahoo.com.tw is failing a validity check somewhere up the food-chain because it sees the *.com and *.tw TLDs as conflicting.
I must try out someone@yahoo.com.cn later ...