You granted send as permissions to an account from Exchange Management Console in exchange 2007 environment and realize that after an hour later the permissions have disappeared, in fact any other explicit permissions that you granted through Active Directory have gone as well. This will also becomes an issue for organizations using BlackBerry.
Above is by design, read this for more details.
To fix the issue:
Remove the account from any of the following group
- Account Operators
- Server Operators
- Print Operators
- Backup Operators
- Domain Admins
- Schema Admins
- Enterprise Admins
- Cert Publishers
If that's not an option then see the work around in the following KB Article.
This applies to any version of Microsoft Exchange.