Using Windows live mail 2012 I can receive all emails and send emails but NOT to btinternet.com email addresses, this has been since April 18th 2016 as successfully sent one the previous day. Copied below but ***** replaces email addresses for privacy as I am a business.

BT Internet is my provider.

If I log onto my email account at webmail I can send to btinternet.com addresses and if I use my gmail account I can send too (both of which I rarely do as I use WLM 99.9% of the time)  but I don't want to have to keep doing that as I want to keep everything in one place ,it takes ages and confuses customers.

I have not changed any setting within WLM and I am getting more and more frustrated. 

The message comes later in the day that the original email was sent gets repeated daily so for each original email I tried to send.

<<<<<<<<<<<

This message was created automatically by mail delivery software.

A message that you sent has not yet been delivered to one or more of its
recipients after more than 48 hours on the queue on mail94.extendcp.co.uk.

The message identifier is:     1auMlF-0003Ns-S2
The subject of the message is: Re: Fraggle
The date of the message is:    Sun, 24 Apr 2016 17:19:50 +0100

The address to which the message has not yet been delivered is: *****   Delay reason: SMTP error from remote mail server after MAIL FROM *****SIZE=3808:
    host mx.bt.lon5.cpcloud.co.uk [65.20.0.49]: 421 Too many messages (1.5.6.1) from 79.170.40.94

No action is required on your part. Delivery attempts will continue for
some time, and this warning may be repeated at intervals if the message
remains undelivered. Eventually the mail delivery software will give up,
and when that happens, the message will be returned to you. >>>>>>>>>>>>