Since around Dec '14, many of the emails I send do not reach the other end. This happens completely at random - whether a new message, a reply or fowarded one. The Sent box shows the email has gone. It doesn't matter how long or short the message or whether there are attachments. I would say that up to 50% of my message don't get through. Of course I have double-double checked the email addresses. Sometimes messages get through to the addresses, sometimes they don't. This is casuing me much annoyance as I often need to send really critical information and then have to chase the recipient to see if they received it. Rubbish!
I've got the same issue - I'm using Outlook on a Windows 7 PC. As far as I know there is no issue for me receiving emails. But when I try to send emails to people on TalkTalk the email either doesn't arrive at the recipient's PC or is very delayed (eg delayed by days). Friends on other email providers (eg virgin) don't have a problem with emails from me. Someone looked at the email headers on my emails and noted that they had one element (amongst a lot of data that I could not interpret) "X-Junkmail-Premium-Raw: score=47/50" which suggests that somewhere between my PC and the recipients PC a junkmail rating has been inserted. So perhaps my emails are being delayed/rejected because something thinks that my email is junk mail ? However even if true it doesn't help me because I don't know where this junk mail rating has been inserted. Oh, and I also tried sending via BTMail webmail and the TalkTalk recipient got that email ok - does this mean that the problem lies with Outlook on my PC ? (ps I have Norton 360 on my PC and it has not reported any problems with viruses etc)
Looking at recent forum threads there are a number of people with similar problems, but BT doesn't seem to be taking the probelm seriously. As far as BT is concerned as long as web mail works that's all that's required. They do not support any e-mail client.
The biggest problem I find - which suggests to me that it is not local client related - is that the problem is intermittent. Some days mails sent via Outlook will arrive other days they will not. I also have my suspicions that webmail is not 100% reliable but with an intermittent problem it is so difficult to be sure.