What is your account type - POP or IMAP? In the latter case the spam filter on the server my be the culprit.

It is IMAP. If it is the server (sky/yahoo) is there anything I can do to correct it? I've looked at the help articles and can find plenty of information about blocking emails, but nothing about unblocking.

this is not blocking. it is spam filtering and it is probably the result of Yahoo choices, not Thunderbird.

Thunderbird will not put an email from someone that is in your chosen address books into the junk folder. Yahoo has no such compunction.

Options > Account settings > Junk settings to ensure the address book the person is in is chosen as one to white list. Otherwise your stuck figuring out the impenetrable wall that is yahoo.

If it is the server (sky/yahoo) is there anything I can do to correct it?

Log in to your account via webmail. In the Webmail GUI go to your Bulk Mail folder. Mark any messages you don't want to see there as 'Not Bulk'.

Whether Yahoo is capable of learning what's spam and what isn't - you'll find out.

Thank you both for your advice. I'll give the yahoo route a try, and if that fails I'll ask the sender to use my other email address!

All of the answers seem to get back to JUNK filtering, but this is the SPAM filter. I also find information on BLOCKING but never un-BLOCKING. At no point did I ever mark these messages as JUNK or set them to be BLOCKED. My mail comes through my personal hosting service, so is not affected by any of the typical Yahoo, AOL, or MS based mail services.