If the mail is placed in the junk folder by your mail providers spam filter and it arrives in Thunderbird via IMAP sync already in that folder, you are probably out of luck. You might try right clicking the folder, selecting properties and turning on the option to get messages for this folder when checking mail. The designated spam folder is not routinely checked for mail on IMAP accounts.

Hi Matt. Thank you for your reply to my query. However the two accounts that do provide me with alerts in my inbox when emails arrive in the spam box are both IMAP - indeed the basic settings in all my accounts (Thunderbird and ISP (1and1/ionos)) are identical, so I had assumed that there was an arcane function deep within Thunderbird that could be set. If that is not the case then it will remain a mystery why the (very useful) feature is present in two of my accounts but not the others. I've checked the properties of all the folders in each of the accounts and they appear identical

Kind Regards TechVet

Hi again Matt: Apologies. I've just discovered that the "Spam Report" that is the alert I get in the inbox, is a utility provided by 1and1/ionos, the ISP. So problem solved. Sorry for putting you to the trouble of an issue which is nothing to do with Thunderbird