Mail delivery delays in queues

server OS = MS 2000 SP4 (all updates)
Exchanges = MSexchange 2000 SP2 (all updates)

Spam messages sit in queues too long delaying delivery of other messages to that domain.

Messages in the queues have an icon on the left which appears to be a yellow envelope incorporating a white circle with a small red square in the bottm LH corner of the envelope.

Messages with this icon seem to stay in the queue for too long thereby preventing delivery of other messages to that domain.

Eventually they disappear.
Is there any way to stop this holding pattern.
Could server memory be a contributing factor? It needs upgrading but other circumstances prevent this for another 12 months.

Many thanks in anticipation