To Delete keychain items on your macOS, please follow the below-mentioned steps:
1) Open Canary Mail and purge. To purge, open Canary Mail > Click on ‘Help’ in the menu bar > Select Purge.
2) Open Keychain Access on your macOS device and search for io.canary.leveldb & io.canary.realm.
3) Delete all the results under these searches. Command + A to select all the results > two-finger tap/right-click > Delete.
4) After successfully deleting the keychain items, re-open Canary Mail.