When Sieve scripts are uploaded using the ManageSieve service, they’re immediately compiled and the script upload will fail if any problems were detected.
Not all problems can be detected at compile time however, so it’s also possible that the Sieve script will fail during runtime.
In this case the errors will be written to the
.dovecot.sieve.log file (right next to the
.dovecot.sieve file itself in user’s home directory).