You do NOT need to delete the existing log files. If you do this you will
throw away all mail sent and received since the last backup was performed.
If you keep the log files, then you can recover back to the position just
before the system was crashed.

The log files work - keep them.

HOWEVER if you do not have all of the log files since the last backup was
performed, then Exchange will not be able to play forward through the log
files. In this case, you WILL need to delete them first.

1. Make sure you read the MS White Paoer on Disaster Recovery of Exchange
systems.
2. Make sure you do NOT have Circular Logging selected.

Cheers, Chris