70,000+ records in OAIB table... with UserSign "-1"

Hi all:

My customer is experiencing slow logon times and I traced it down to large OAIB and OALR tables with more than 70,000 records. The strange thing is that 99% of the messages have a UserSign of "-1" in OAIB and thus I can't ask any specific user to delete those messages. This has happened before and I have users pretty well trained in deleting old messages from their inbox, but I'm not sure what the messages sent to user "-1" mean, why they are there and whether they could be safely deleted...

Thanks for any insight...

Joerg.

This is the query showing the number of messages:

SELECT
	COUNT(T0.usersign),
	T0.userSign,
	T1.u_name
FROM
	OAIB T0
	LEFT JOIN OUSR T1 ON T0.usersign = T1.Internal_K
GROUP BY
	T0.usersign,
	T1.u_name