I thought a similar problem for some weeks with delayed inbound and outbound emails was because of something on my computer or a hiccup at my ISP - I'm not with BT - so I rang my ISP (Eclipse Internet) and they immediately asked if the problem was only with btinternet and btopenworld addresses. I confirmed it was so it seems BT have a problem.
For example a friend with a btinternet address sent me an email on Thursday 12th and it arrived on Tuesday 17th. I replied and got this message:
This is an automatically generated Delivery Status Notification. THIS IS A WARNING MESSAGE ONLY. YOU DO NOT NEED TO RESEND YOUR MESSAGE. Delivery to the following recipients has been delayed. <[email protected]> The reason for the problem: 4.3.2 - Not accepting messages at this time 421-'Too many messages (1.5.7.3)'
He eventually got my reply two days later! That warning message was not an isolated one - emails to several btinternet friends came up with the same message.