Do not use Notes client single logon.

IBM has issued a Security Bulletin for Notes Client single logon feature.

Notes Client Single Logon uses an operating system communication mechanism for password transmission between Windows and Notes that can be attacked by malicious code planted on the user workstation to reveal the user password. To prevent the potential for attack, disable Notes Client Single Logon and instead opt to use the more secure Notes Shared Login or Notes Federated Login.

For more details see here