Problems with emails to ‘@btinternet’ addresses?

[#1]

This is not a PlusNet problem but I am wondering if anyone else has recently encountered the issue described below.
When sending emails to an ‘@btinternet’ address yesterday they were ‘bounced’ and I received the following non-delivery error:
SMTP error from remote mailer after end of data:
    host mx2.bt.mail.yahoo.com [217.146.188.189]:
    554 Message not allowed - UP Email not accepted for policy reasons.  Please visit http://help.yahoo.com/help/us/mail/defer/defer-04.html[120]
The emails concerned were received by other recipients but not by the ‘@btinternet’ account.  Previous emails, in March and earlier, were accepted by this account without any problems.  As a test, I used both Outlook Express and Thunderbird as the email client but the messages failed regardless of which one was used.
I checked the web and it seems that BT or Yahoo or both have made some changes recently which may have caused this to happen. 
The following article refers to recent changes made by BT which is affecting certain BT customers who use their own domain names for email rather than a recognised domain name in BT’s whitelist.  http://www.theregister.co.uk/2008/03/25/bt_email_verification_update/
I am a PlusNet customer, not a BT customer, so I am on the ‘outside’ trying to email in to the BT domain.  The above change, in theory, should not affect my email.  I have my own domain name which is used for email and I wonder if this tightening of policy, or any associated changes, is the cause of my messages being bounced.

[#2]

Hmm strange, looks like the email is being deferred because there is something unacceptable within it. If there is a link in the email try removing it and sending again to see if this goes through.

[#3]

Thanks for your suggestion Chris.
The email was about Internet Security packages and did contain URLs to various websites, Symantec, ESET, etc.. Two of the URLs began with ‘https//’.  As a test, I’ll take out the URLs and try again.  It’ll be very odd if it works!

[#4]

That was just a suggestion from a quick Google search of the error message you are getting, seems to have worked for some people.

[#5]

Taking out the URLs worked.  After doing so, I didn't get a non-delivery report and I've now had confirmation that the message was received.  Very strange!  Thanks again Chris.