I've struggled with sending mail to multiple recepients: I did not receive a mail when more than one recepient was found.
Seems the problem was caused by using a BCC in combination with multiple recepients...

If you want to simulate:
A. create an e-mail Notification "testMail" with:

BCC: some.one@domain.com

B. create a rule to send the mail to multiple intelligent BO's

FIND RegularUser WHERE RegularUser.EmailAddress IS DEFINED
SEND testMail TO RegularUser

Test-scenario's:

1. Run the process with only 1 RegularUser having an e-mail address: OK, mail is sent
2. Run the process with only 2 RegularUsers having an e-mail address: mail not sent, error in Aware IM Server Output:

2011-04-29 09:46:41,774 ERROR Beleidscyclus_BM#awareim -Exception sending an e-mail.

java.lang.ArrayIndexOutOfBoundsException
	at java.lang.System.arraycopy(Native Method)
	at com.bas.basserver.iopipes.EmailSink.a(Unknown Source)
	at com.bas.basserver.iopipes.EmailSink.processMessage(Unknown Source)
	at org.openadaptor.adaptor.STPipelineController.processMessage(Unknown Source)
	at org.openadaptor.adaptor.SimpleController.processMessage(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimpleSource.processMessage(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimpleSource.sourceProcess(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimpleSource.sourceProcess(Unknown Source)
	at com.bas.connectionserver.iopipes.RequestReplySource.sendMessages(Unknown Source)
	at com.bas.basserver.channels.ChannelManager.a(Unknown Source)
	at com.bas.basserver.channels.ChannelManager.sendNotifications(Unknown Source)
	at com.bas.basserver.bsmanager.f.a(Unknown Source)
	at com.bas.basserver.bsmanager.e.a(Unknown Source)
	at com.bas.basserver.bsmanager.f.a(Unknown Source)
	at com.bas.basserver.bsmanager.f.a(Unknown Source)
	at com.bas.basserver.iopipes.ComponentSink.processMessage(Unknown Source)
	at org.openadaptor.adaptor.STPipelineController.processMessage(Unknown Source)
	at com.bas.openadaptor.NonTransactionalController.processMessage(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimplePipe.processMessage(Unknown Source)
	at org.openadaptor.adaptor.STPipelineController.processMessage(Unknown Source)
	at com.bas.openadaptor.NonTransactionalController.processMessage(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimpleSource.processMessage(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimpleSource.sourceProcess(Unknown Source)
	at org.openadaptor.adaptor.AbstractSimpleSource.sourceProcess(Unknown Source)
	at com.bas.openadaptor.a$a.run(Unknown Source)
	at java.lang.Thread.run(Thread.java:619)

If you're doing the same, but using CC and not BCC --> everything OK

So, beware of using BCC 😉