Sending an email to someone using the Outlook Auto-Complete List may be returned as undeliverable or if you send an attachment the attachment may be dropped. A work around is to remove the address that is being reported as invalid from the Auto-Complete List. A more destructive fix is to empty the Auto-Complete list. Emptying the Auto-Complete list means that when you type an address or name in the 'To' field it will not Auto-Complete based on your previous emails. If you empty the Auto-Complete list it will 'relearn' as you continue to use Outlook.
How to remove an address from the Auto-Complete List:
Step 1
Open a new email and begin typing the email address you want to remove in the 'To' field
Step 2
Use the up and down arrow keys to select the address to be removed. When the address to be removed is highlighted click the 'x' icon beside the address to remove the entry
How to empty the Auto-Complete List:
Step 1
Click on File, then click on Options
Step 2
Click the Mail tab on the left and scroll to the 'Send Messages' section
Step 3
Click 'Empty Auto-Complete List' button
Step 4
A confirmation box will display, click Yes