For me this happens when Outlook for Mac 2016 tries to download attachments using a weak internet connection. Somehow, the download fails, and Outlook is unable to re-download later when a better connection is available. So the greyed out attachments are just stuck in limbo - can't open, can't redownload. Outlook and Outlook for Mac: Update File Versions This Wiki page lists the MSI-based Outlook related file versions and updates with release dates and KB articles. Click an Outlook.
-->Original KB number: 2648915
Summary
This article contains information about the compatibility of Microsoft Outlook for Mac and Apple iCloud.
Outlook for Mac does not support Apple iCloud calendar (CalDAV) and contact (CardDAV) synchronization.
Outlook for Mac does support iCloud Mail. For steps on how to configure your iCloud email account in Outlook for Mac, go to the More Information section of this article.
More information
To configure your Apple iCloud email account in Microsoft Outlook for Mac, follow these steps:
Start Outlook for Mac.
On the Tools menu, select Accounts.
Select the plus (+) sign in the lower-left corner, and then select Other E-mail.
Enter your E-mail Address and Password, and then select Add Account.
Note
The new account will appear in the left navigation pane of the Accounts dialog box. The Server Information may be automatically entered. If not, follow the remaining steps to enter this information manually.
Enter one of the following in the Incoming server box:
imap.mail.me.com
(forme.com
email addresses)mail.mac.com
(formac.com
email addresses)imap.mail.me.com
(foricloud.com
email addresses)
Select Use SSL to connect (recommended) under the Incoming server box.
Enter one of the following in the Outgoing server box:
smtp.mail.me.com
(forme.com
email addresses)smtp.mac.com
(formac.com
email addresses)smtp.mail.me.com
(foricloud.com
email addresses)
Select Use SSL to connect (recommended) under the Outgoing server box.
Select Override default port under the Outgoing server box, and then enter 587 in the Outgoing server port text box.
After you have entered the incoming and outgoing server information, Outlook for Mac will start to receive your email messages.
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Note
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You can select Advanced to enter additional settings, such as leaving a copy of each message on the server.