Unable to send mails to btinternet addresses

[#1]

Looking through previous posts I see this has been a regular problem.  None of the posts I have seen are the same as my problem however.   I have a group of 14 people set up under Outlook 365.  If I send a message to this group it gets to 10 for the group but not the other 4, all of whom have btinternet addresses.  I can take the same message and copy it into gmail and send from there and it gets through with no problem.  I have tried sending individual messages and simple test messages but nothing works.  About 4 days letter I get a reply saying the message could not be delivered.   I have tried from two different user names on my PC and so two different Outlook settings, and from my Windows 10 laptop, none of which are successful.  This started suddenly about 5 or 6 weeks ago.  On 19th May I had a chat session with a plusnet advisor who could not help but said he would pass it on to the email specialists.  Last week I phones an advisor with a somewhat similar response.  Neither of these has solved my problem.   Does anybody have any idea how I can resolve this, other than changing ISP.

[#2]

Welcome to our community forums @IanHill, sorry to hear you're experiencing problems with emails making it through to certain contacts.

So we can look in to this further, could you provide us with the full bounce back message you're receiving and the email headers.

Feel free to attach these to the ticket on your account regarding this matter Here, as I've escalated this to my team now to monitor and get back to you once we can look in to this further for you.

Thanks.

[#3]

I think I remember something very similar happening to me earlier this year, after btinternet started being more restrictive about what they would accept in incoming emails. After some trial and error I found that btinternet was  objecting to my "signature" containing a link. It was innocent enough: if someone clicked on my name they were taken to my personal website, but it was making btinternet suspicious. As soon as I removed that link all was well again. Might that apply in your case too?

[#4]

Sadly not the same as my experience.  I have tried sending the simplest of messages, namely a message titled Test and the text Test message.  No links of any sort and it still bounces, coming back with the messages unable to deliver after repeated attempts.  Thank you anyway

[#5]

Thanks for the info @IanHill

We'll need to look into this and we'll get back to you next week.