I am having a problem with users trying to send from other users. The users wish to send from 3 domains. I wasn't sure how to do this and I don't know if I did this correctly but it worked for a little while then it stopped. I created additional users with email addresses from Domain B and Domain C. This worked once I gave the users 'Send As' rights. Then a few days later, the users couldn't send from Domain B. I noticed there user accounts that have populated a bunch of users from Administrator, Anonymous logon, Self, etc on the Mailbox Rights for Domain B but not for Domain C. Do user rights automatically get assigned to a mailbox? Nobody has assigned rights to these accounts. I asked the customer if they did anything different such as logging on but they said they haven't done anything. I'm confused. Any feedback would be greatly appreciated, or if you have a better solution on how to send from other domains please let me know. Thanks!

A newmailbox is instantiated by Exchange when events like the first user client logon or the first message delivery. At this point the mailbox internet permission from parent objects in the local domain. You would have to assign specific permissions to security objects in other domains on each account manually.