We’ve not had the exact same issue as you but something with Outlook. We have just upgraded our only Mac to Big Sur, installed KES 11.1.0.210 & Outlook 2019. Outlook connects fine and send’s and receives emails, but as soon as you send an email with a large attachment (25mb for example) it will just not send and causes an Outbox block. As soon as you quit KES, it will send. I have also tried disabling components of KES (1 by 1) etc and it didn’t make a difference. I have a ticket opened this morning under INC000012188263 I did want to try adding Outlook to the trusted apps, but I don’t know where to find the paths required.