Well, having said it is only BT email addresses bouncing my emails I just got a non-BT one...
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
host cmailcluster.ourit.co.uk [188.8.131.52]
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>:
554 5.7.1 Service unavailable; Client host [xx.xx.xx.xx] blocked using zen.spamhaus.org
Further testing of this issue this morning reveals the following.
I have reset my hub numerous times to get different ip addresses. Every one has been blacklisted on at least two and sometimes four antispam services. I note that SPAMHAUS say this does not stop emails being sent.
The only email addresses that my emails are being bounced by other contacts with btinternet email addresses. So it would seem only BT are blocking emails flagged as from their own blocklisted ip addresses.
How daft is that!
This is not daft, this is by design. ALL of BTinternet ISP dynamic IP addresses - the ones that end users have - are blocklisted. This is to prevent spam by infection of the end user equipment. In the "olden days", viruses set up their own sending engine using the IP address of the user to send spam emails. That doesn't really happen anymore, but the IP addresses are still blocked for this reason. What should NOT be blocked is the BTinternet email server addresses - in other words people sending emails via webmail and clients using those server addresses, in this case mail.btinternet.com
Sometimes the recipient doesn't like certain ISPs - and can block them willy nilly, which is their choice.
You said that you can only email people with btinternet email address, so are you saying that emailing anyone or any other company always fails? If so - has this always been the case, or has this just started?
No, I said "The only email addresses that my emails are being bounced by other contacts with btinternet email addresses."
So emails sent to those with btinternet email addresses are those that most often fail, and currently always fail.
BT's receiving email service is stopping emails from a legitimate source.
Such failures have been occasional in the past but currently frequent.
Is it a BT email address that you are sending from or is it another email address?
Not from a BT email address.
If you are sending from your own domain I suggest that you have a read through this link and also contact your hosting company and ask them to ensure that they are complying with the information in the link.
BT Email - best practices for postmasters and senders of email | BT Help
In case you are sending from your own domain name… and all depending on how you are doing this… measures can be taken to prevent being seen as a ‘potential’ spammer.
If you can, your domain name should have a reverse DNS (rDNS) so yourdomain.com points to 184.108.40.206 for example, and would also need the record of 220.127.116.11 pointing to yourdomain.com — domain readbacks used to be very common and there used to always be an issue sending to/from AOL email addresses. Annoying.
These days an SPF TXT record can be set, which should be provided by your email provider and added in your DNS at your host. This acts in much the same way basically authenticating that you are not spoofing the domain name.
It might not be BT being the problem, but just an email server not quite meeting all the criteria that validates it as legit.
If not using your own domain name then disregard the above.