This happened sometime ago but has just re-started.
I'm on BT Mail (not Yahoo). My mail goes via my 1&1 email address and is then forwarded to a BT mail address. This allows me to have a personalised email address. It is plain ordinary mail and the bounce backs occur randomly. I suspect that it's filters on BT Mail that treat, occasionally, email from 1&1 as some kind of bulk mail spam. If I were guessing it will be when there's a good deal of traffic coming via 1&1 to BT addresses, hence the apparent random nature. This does not happen with plain vanilla btinternet addresses.
It happened a few months back and I thought had been sorted but it's creeping back again. I know this because I get the same mail sent to a Hotmail address that receives it all, no problems. I set this up as a defence against BT's poorly configured mail servers and I'm glad I did! BUT it is important that BT aren't let off the hook and sort this once and for all. I'd really like to talk to the email people in BT but ringing 150 doesn't let you anywhere near them hence the pleas here. Perhaps a moderator could help?
I had something similar with TalkTalk last year. In my case those with TalkTalk email didn't receive my email but other addressees did receive ok. One of my addressees looked at my email with "view Source" and noted that the email head had a high spam score (so might be rejected) - I haven't a clue why my emails (BTMail) were marked like that because there was nothing in the content that anyone could object to. As an experiment I then sent a webmail (instead of using my Outlook mail) and the TalkTalk addressees got that ok. So I concluded that the BT screening system didn't like my Outlook email for reasons unknown to me. After a few weeks this problem disappeared and for at least a year I have been able to send my emails using Outlook without any problems ! So .... try a wemail (are you using Outlook for your email ?)
An email from Wed 13/04/2016 14:21 has just arrived only three days late. Come on BT Mail sort this out! It's worse than snailmail!
This sounds like the same problem as this thread https://community.bt.com/t5/Email/Domain-email-to-Spam/td-p/1605475/page/5
Interesting to hear it is affecting other hosts than fasthosts. I has to be a BT problem
I've had an apology from BT Mail about emails going missing yesterday (see at bottom).
I replied but of course it wasn't a real address so bounced. I'll post the reply here so the MODERATORS can pass it to the email team (some hope!):
MY REPLY THAT WAS BOUNCED BACK
Thank you for the apology - some progress.
This has been happening more than once without any such apology. For example, an email sent to this address was sent on 13/04/2016 at 1421 and didn't arrive until 16/04/16 at around 1500. I can give other examples.
I think this is happening with my emails that are forwarded via a 1&1 Domain. I'd like this domain whitelisted to ensure my mail gets through. Currently I'm also forwarding some of these emails to a Hotmail address and that is receiving them in good time. This is not a solution to BT Mails high level "filters" that stop legitimate mail. I set up this second forwarding address over a year ago when similar problems arose without any acknowledgement from your good selves.
Sounds very much like the problem I'm having intermittently with BTYahoo. If I send an email to myself at my main btinternet.com address it can take days to arrive - if it ever does arrive at all! If I send the same message to myself at one of my subaccount email addresses it arrives straight away, no problem. Anyone else having this problem? Anyone know a fix?