Hi Aidan.

Blocking is not possible in Thunderbird. You may be able to do it at the server.

What you can do is Thunderbird, as you seem to know, is filter messages to trash, junk, or somewhere else. To start creating a message filter, use Tools (menu) > Message Filters.

You can access the same function through the three horizontal lines on the toolbar.

Select a message, then Message/Create Filter From Message... Press Alt if the Menu Bar with Message is hidden.

But it's better to block (or whitelist) senders through the options offered in the webmail settings for the account - which are applied even when TB is closed or before mail is downloaded.

Thanks for the replies. I'll certainly have a go at using filters and seeing how easy it is to block emails at web mail level.