I will try to summarise this without it getting confusing. This is partly Outlook and partly Exchange so if I need to move this question to another forum, I apologise and will do so....
FYI, we are in a co-existence Exchange 2003/Exchange 2010 environment, although all the concerned parties are on Exchange 2010.
UserA attempts to send an e-mail "From" ResourceA mailbox to Distribution List "DL-A". UserA has been delegated Send As permissions to ResourceA through the Exchange 2010 EMC.
Also in the EMC, DL-A has a Message Delivery Restriction (Properties -> Mail Flow Settings -> Message Delivery Restriction) Accept messages from - Only senders in the following list. This list contains ResourceA.
When UserA attempts to send to DL-A when ResourceA is in the "From" field in Outlook, she is unable to send the e-mail and receives a bounce. In Outlook 2010, the pop-up rules also state "You don't have permission to send to: DL-A". If UserA is added into the "Accept messages from" restriction on DL-A directly, she is (obviously) able to send to this DL without the pop-up or the e-mail bouncing.
Could someone please explain why this is occurring, and if any workaround is possible?
More information can be provided if required.
Thanks,
Chris