Page Comparison

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Method 1

Re-Sync the Offline Address Book (OAB)

1. Click the Send/Receive tab.
2. Click Send/Receive Groups. A drop-down will appear.
3. Click Download Address Book.
4. Uncheck "Download changes since last Send/Receive"
5. In the "Choose address book:" drop-down, select \Offline Global Address List (it may be set to that by default).
6. Hit OK
7. Wait for it to sync. 
8. Complete the Clear Autocomplete Data steps below and re-test.

Clear the Autocomplete Data

1. Start a new email
2. Click From. In the drop-down button menu there will be an autocomplete entry with the other email address.
3. Click the X to remove any mailbox entries that are experiencing undeliverable errors.
4. Click Other Email Addresses to look it up in the address book and select it from there. You will need to do this and send an email to every address experiencing the issue, but after that you should be able to use the Auto-Complete suggestions again.

Method 2

Reload the Email Data

1. Make sure Outlook is closed.
2. Search Windows for Control Panel, and click to open
3. Click Mail (Microsoft Outlook 2016)
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4. Click Data Files
5. Select your email account from the list and click Open File Location...
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6. In the Explorer window that pops up showing the email file's location, at the top, select the View tab.
7. Make sure File name extensions is checked:
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   Your OST file looks something like this:
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8. Rename your OST file to .old (i.e: example@otc.edu.old)
9. Relaunch Outlook. A new OST file will be generated. It may take some time for all of your email to re-sync.