Account keeps on locking


Solid State Member

I have a dell optiplex gx280 with windows xp, office 2003, internet access. Problem is everytime user send mail from outlook-his account gets lockout, he can read mail but when replying or sending new messages his account gets locked out, Checked eventviewer-this event is showing at the time the account locked. 4-6 entries for just that day when account locked a day(4-6 times)

Source: LSasrv
Category: SPNEGO (Negotiator)
Event Id: 40960
The Security System detected an attempted downgrade attack for server exchangeAB/ The failure code from authentication protocol Kerberos was "The user account has been automatically locked because too many invalid logon attempts or password change attempts have been requested.

What can i do to fix this??