Hi I assume you have already contacted support at BT, your email service provider, and possibly also your Internet service provider? If (one or several) of their servers have been blacklisted, there is not much you can do except let them know. There might also be some info at https://community.bt.com

The error message suggests you try sending your email from Webmail - did you try doing that, and what happened? I understand the issue is intermittent, so you might need to send several messages using Webmail to be sure whether that method is also affected. Trying the same outgoing messages through Webmail is an excellent test to see whether the problem is entirely to do with your internet and email service. If you get the same effects while using webmail as you do when using Thunderbird on your local machine, then the problem definitely lies with your service providers and not with Thunderbird.

You addressed this to "Postmaster" - so I just want to verify you know list is not for Postmaster support. It is for Thunderbird support, provided by volunteers around the world. Best regards, Agnes

Hi Agnes

Thanks for the reply. A support request has been raised with BT. Postmaster in this case is the BT mail administrator. BT is also the internet service provider in this case. I have used the web based mail interface from BT and this has functioned without the issue I am raising being observed. I can find no information that is related in the link https://community.bt.com. Hence, from the information that I have, it appears that the issue is likely to be in Thunderbird.

A defect in BT's mail system cannot be ruled out at this stage as the testing I have done has not been near the volume that has been generated via Thunderbird.

Thanks again, Graham

The defect is in bt;s mail system. they are using a blacklist of known spamming ip addresses. (that is usually allocated by BT when you connect to the internet) these blacklists are sometime well known from groups like spamhaus. Some are from other commercial interests and some are just rubbish the provider has cobbled together from a variety of sources.

What is happening is you are being allocated an IP address by BT that is on one of those blacklists. You connect to BT's server to send mail and it refused because the IP address issued by BT is on a blacklist. So best get onto BT and ask them to fix the issue. Perhaps introduce their right hand to their left.

Thanks very much for the insight, I will pursue BT for a resolution.

I received the same message and contacted BT who suggested use their webmail which was satisfactory and no problems