Username and password keeps popping up when users send email.

Postfix, SASL with MySQL
O/S Debian 3.1 Sarge
System: 1 Gig Memory, Dual PIII 900

We currently have about 500 email accounts using postfix/mysql and most of the time email sends fine without a problem. However we are having an issue with username and password prompting users a few times a day when sending email.

I have been monitoring the load on the server using top and normally load is between 0.02 1.65 (I have been told SASL will stop when load above 2.00 (Can this be increased))

I have also noticed this happened when load was only 0.12 so it maybe some maximum connections setting in mysql or sasl.

Does any one know what may cause this.

Regards
Michael Taylor