Send as doesn't work

I have 3 users that need acces to "send as" I gave them these rights on the particular mailboxes.
I always get the error that she/he doesn't have rights to send to that person.
ex:
First mailbox is info with 2 other mailboxes (one under-cover.be and one 2lite.be)
If I try to send as the 2lite or under-cover the mail always comes back with the message not enough rights to send to "that" mailbox

Any ideas?
Info-ServiceAsked:
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Delphineous SilverwingGood Ol' GeekCommented:
Please clarify - your example says that the sender does not have rights to send TO the mailbox.

Have you inadvertently configured a "Message Restriction" to Accept messages only from certain people (in Exchange General > Delivery Restrictions) instead of "Send on behalf" (in Exchange General > Delivery Options)?
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Info-ServiceAuthor Commented:
I don't want to send on behalf. I want to send as...
I gave the users that need to send as the rights (via the AD users, security tab, add users, send as box ticked)
I get the error when I send internal mail as external...
And indeed i get a message that i cannot send TO that mailadress, strange.
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Delphineous SilverwingGood Ol' GeekCommented:
Have you given them Full mailbox access (Exchange Advanced > Mailbox Rights)?
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Info-ServiceAuthor Commented:
yes
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Delphineous SilverwingGood Ol' GeekCommented:
Go into the Security tab of the AD account properties and make sure the user has "Send As" permissions for the account.  (This may require you to View > Advanced Features in AD Users and Computer)
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