Sophos Email Advanced, while their are many email quarantied as spam their is also email that are 100% spam gets delivered to users.
Please make sure that you have followed the recommended settings under the Anti Spam Filtering in Sophos Central." It was noted that email message is analyzed and given a spam score. The higher the score the more likely the message is to be spam.
Messages with the highest spam scores are rated as Spam."https://support.sophos.com/support/s/article/KB-000038822?language=en_US
If it still being delivered even if you have followed the article, I would suggest to get at least one copy of the email and forward to the Sophos Lab for them to confirm if this is a valid to be detected or not. Please follow the kb article below to submit the sample spam directly to Sophos Lab.https://support.sophos.com/support/s/article/KB-000033422?language=en_US
Thanks and have a nice day!