No transport provider was available

No transport provider was available for delivery to this recipient.

We are recieving this sporatically throughout our enviroment. It ussually occurs when users try to send to multiple recipients that have been selected from the address book. Whenthey try the very same addresses typed in and sent seperately, mail goes fine.

We are approximately a 300 user enviroment using a Pop mail server, that is supported by a 3rd party, running Sendmail. We are a mixed 98, 2000, XP os desktop enviroment with various “Microsoft Office” suites installed.

Any help will be appreciated