If you have a specific domain (or domains that you want to reject you could:Go to: Admin panel -> Manage -> Ticket Filters
Click Add New FilterFill in "Filter Name".Fill in Execution Order: 1Set Filter Status: ActiveSet Channel Target: EmailsMake sure Rules Matching Criteria: is set to Match Any.Under Filter Rules do the following: In the first row, Set the first column to "User / Email Address".In the first row, Set the next column to "Equal"In the first row, Set the third column to "xyz.sk"In the second row, Set the first column to "User / Email Address".In the second row, Set the next column to "Equal"In the second row, Set the third column to "xyz.ck"Tick "Reject Ticket".Click Add Filter down at the bottom.