Are the messages bouncing back to you? If so could you copy and paste the content of the bounced message?
Are you trying to send to more than one person at a time? Also is there any more information in the returned email, usually the indication of a reason is in there.
Lastly is this happening only from your laptop or phone or does it happen on both?
Ok then most likely you may have been blacklisted from sending to btinternet for some reason. One moment, just need to check something but has anything odd happened with your outlook account lately? Sometimes a hacker will break into your account and send a bunch of spam causing the black list.
Yes that is why I am thinking your outlook account may have been blacklisted. Can you check your outlook sent mail folder and see if there is anything in there that you do not recognize?
Good. This may be an outage on BT's end, I am just checking with them right now. One moment.
Ok I can see that others have been having issues with BT for over a week now https://downtoday.co.uk/bt-email/
This really seems like they are having issues on their end which is likely what is causing you not being able to send into their network. Probably the best thing to do for the time being is to send to your BT users only from your BT account until this is sorted.
The issue cannot be fixed from your end since it is their problem. Aside from using your BT account you can email the BT postmaster directly and see why your messages are not getting through. Send an email to firstname.lastname@example.org
They will be able to see if your mail server was blacklisted or if this is just a problem on their end for sure. But from what I have found it looks like an issue on their end that they are working on.
Email issues like this cannot ever be corrected on the senders side unless you were blacklisted which means you need to email the postmaster to get off the blacklist. But based on what you have told me it does not sound like you have done anything to warrant blacklisting, usually you get blacklisted for sending many emails (spam) into BT's network and does not sound like you did that.
So unfortunately all you can do at this time is use your BT address to send to BT people and let BT sort it out. If you want to be sure it is an outage send an email to the postmaster address I gave you just to be sure however I would not expect a quick reply. Also if you are a current paying BT subscriber you can contact them here:
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If you had a problem that was I could solve from your end I would solve it. Now given the situation and the information you provided I gave you the best answer I could. We could always do a remote session and I could further confirm the issue and make sure 100% that the issue is on their end however that would require payment up front as I need to get paid for my time. I have been paid nothing for the information I have given you yet. You have placed a refundable deposit but no funds have been transferred to me and will not be unless you rate my service 3 stars or more.
I am happy to remote into your computer and check it out first hand if you would like but I would need to be paid for my time and we still will probably end up with the same result. I can see BT has been having issues for over a week and there is nothing anyone aside from BT can do about that as I am sure you understand.