I presume you are using Thunderbird on the iMac? Check that you have subscribed to the "spam" folder. "Spam" is a word that Thunderbird doesn't use itself, so we're talking about spam detection and quarantine that is being run by your email provider. They are identifying your probable spam messages and moving them to a separate folder, and my guess is that Thunderbird isn't set to display this folder. In the main Thunderbird menu, look under '''File|Subscribe''', select the account and check if its "Spam" folder has been selected.