One HealthMailbox keeps on logon failure

We have 2 exchange servers, each has all roles including 1 mailbox database. We also have 1 DAG. Of course Exch1 is the primary.

We have 23 healthmailboxes: 10 as HealthMailbox-Exch1-001/010, 10 as HealthMailbox-Exch2-001/010. and 3 other HealthMailbox-(long string).

We understand 1 healthmailbox for each database, 10 for each CAS. 

Q1: why we got the 3rd HealthMaibox-(long string)? 

We always experienced HealthMaibox-Exch2-001 keeps on logon failure according to ADAudit Plus report. We have deleted all healthmailboxes and restarted Health manager to recreate them. Always the same result.

Q2: why we got logon failure on the healthmailbox, even on the first day of recreation? We dont have any GPO for force pswd change, and further more by default all healthmailboxes are "pswd never expired"

Thanks for help.

GPING