What to do it you are experiencing Undeliverable Email Notices (aka: Bounce Backs)
When experiencing Undeliverable Email Notices (aka: Bounce Backs) please complete the following steps:
Begin to enter the name of the individual/inbox you’d like to send an email to; once the name is displayed in the cached “recent sender” list, click the X to the right of the name.
This will remove the email address from your cached recent senders.
Once the cached address has been removed, you may click “To” to bring up the Global Address Book
Locate the individual/inbox you’d like to send to and double-click their name and click “Ok”
You may now compose and send your email as normal.