My internet went down and it caused mail too stop responding and required a force quit. Took a while to determine that the internet outage was causing Spam Seive to freeze the mail app.
Thanks for the report. I’ll see if I can fix this in the next version. In the meantime, if there’s an outage, you can temporarily turn off filtering by selecting Disable spam filtering in Apple Mail in the Settings:
A response this fast is customer service at its best. Thank You! I have been using Spam Sieve for several years.