Thunderbird prefixes mail from BT yahoo email address with [SPAM]

Question

When I open Thunderbird to read mail from BT yahoo, the mail is prefixed with [SPAM]. This has happened in the last few days, BT maintain it is nothing to do with them and to my knowledge nothing has changed in the last months. The messages are not shown as spam in BT yahoo mail.
Whislt talking to BT they sent me a mail which (as Thunderbird was opened) was received without the [SPAM] prefix and this morning an Amazon email was received whilst Thunderbird was open, again without the [SPAM] prefix.
It does not do this with mail from an Orange/EE email address.
Can I prevent this, please? (It is just annoying rather than critical and to me unfathomable)!
This began on 19/10/14 after I had not used my computer for 5 days and I opened all 3 email addresses at once through Thunderbird (as pr normal) to receive 60 emails all prefixed [SPAM] - previously all was well - and BT had still filtered out the mails that I regarded as spam.

Answer 1

Thunderbird does not use the word 'Spam', it uses the word 'Junk'.
So if you are seeing 'Spam' then this is either being applied by the server or you have a program installed on your computer which is doing it. But it is not Thunderbird.
I also use BT and it puts some emails in the Spam folder on webmail account.
But if I get emails in Thunderbird with a 'Spam' entered at the start of the subject line then it has been put there by my anti-virus/firewall program.
For example, I use Norton Anti-virus/firewall product and Norton will add 'Norton AntiSpam' to emails it thinks is spam. There is the facility to place exclusions in Norton.
Other products such as 'spampal' or 'spamassassin' will also add the word Spam.
So I would suspect that you have a program running that is doing this.