Error - Unable to redirect failover from ServerName!mail\UserName.nsf

I have one primary server S1 and one additional server S2, both are added in one cluster. When I make S1 down I expect that notes client request should go to S2 as a part of failover. However when I try to open mail box using client I get following error message on S2 server console:Unable to redirect failover from ServerName!!mail\UserName.nsf

Cluster replication is working as expected without any error. I have also created ‘mail’ replicas and sent on S2 from S1.

Also, S2 is not listed in list shown in Administrator, File>Cluster Directory>Databases by Server.

Please help.