Prohibit send setting issue...

I have 5 different Mailbox database stores with different mailbox limits.  The Mailbox database store  for "Mail401" has the following limits;

• Issue a warning at .2 GB (200mb)
• Prohibit send at .22 GB (220mb)
• Prohibit send and receive at unlimited (need users to always receive email)

I thought all was working by design but it turns out that users in this database are still able to send even though they've reached the "Prohibit send" limit.  What do I need to do to resolve this?

PennyM