If you are trying to add any user under the message delivery restriction and getting below error you probably want to use the exchange shell  in 2010 to add/remove users from certain distribution groups. It usually throw this error message if you have an invalid user object available on the senders list.

Couldn’t find  object “UserObject Path”. Please make sure that it was spelled correctly or specify  a different object.

In-order to tackle this you can use below exchange shell commands.

1) This will allow you to view all the users who are allowed to send email to a particular distribution group. Get-DistributionGroup -Identity “MarketingUsers” | select -Expand AcceptMessagesOnlyFrom | ft Name    

2) This command will allow you to add a particular user to the accepted list. Get-DistributionGroup -Identity “MarketingUsers” | Set-DistributionGroup -AcceptMessagesOnlyFrom @{Add=”Renjith”}

3) This command will allow you to remove any invalid user entries. Get-DistributionGroup -Identity “MarketingUsers” | Set-DistributionGroup -AcceptMessagesOnlyFrom @{Remove=”Renjith”}