Question about Outlook Express

I have several groups set up in WLM (I use Windows 7). The sizes range from 6 to over 70. Until very recently I have had no problems sending emails to these groups. However, I have been away for a couple of weeks and since returning have been unable to send to the groups with 70 members. I get  an error message relating to the server of a particular recipient, quoting error message 452.

On researching this it would appear that the recipient whose address in each case is blocking the sending, is number 50 on the list; implying that WLM has put a restriction on the maximum number I can have in a group. I contacted my ISP (BT) who said they had made no changes to their operation, and, anyway, they could only speak about using the web based BT Yahoo. They suggested contacting Microsoft, hence this post.

The only other piece of information I should add, though I don't really think it relevant, is that whilst on holiday I was accessing my emails using a Google Nexus tablet. I then downloaded all my remaining emails and sent items on to my desktop PC once home.

Can anyone help with this, please?