I have recently been getting error messages when sending emails to 5 or 6 people mainly with btinternet but also talk21 addresses. 

The error messages are 

1.  Error, Address: xx@btinternet.com>, ResponseCode 421, 4.7.0 Temporarily Deferred, 

2. Error, Address: xx@btinternet.com>, ResponseCode 421, Too many messages (1.5.6.2) from 74.201.84.163] 

then eventually 

3.ERROR_CODE :421, ERROR_CODE :Too many messages (1.5.6.2) from 74.201.84.163 

I'm a private user and don't send many emails. Sending emails to 20 recipients would be very unusual so it can't be that I'm sending more than my daily quota, and it is only btinternet and talk21 recipients that are having the problem..