I just can’t imagine the reason why with a simple antispam i must reboot my computer :).
I’m going to take a guess that it is because it creates a proxy between your email server and client, to intercept the email in order for its authentication response to function.
It may just be an “simple antispam” program, but it is a real software. 
I’m betting it’s because of the interface with email app/settings/server.
LM