
I tried removing their Gmail account from Outlook and re-adding it, and that didn't fix it for their Gmail account I'm assuming it would be the same for their other accounts. They can log into the Gmail web interface and upload attachments fine from that. Or drag an attachment from the message to the desktop to save it there. Right click Outlook and select Show Package Contents. Close Outlook once the deleting of account is done. Confirm the action and select delete when it asks for confirmation. You can also select a message, then choose File > Save Attachments. Select the account that is having this issue and select the minus (-) symbol. Click the Attachment button that appears, click the name of an attachment or choose Save All, then choose a location. I did notice they have Norton on there, and I'm starting to think this is an issue dealing with that, but I'm not familiar enough with that to really say definitively. In the Mail app on your Mac, move the pointer over the header of a message. They can click the "Always trust" box, but it seems to uncheck itself. The attachment previewer feature in Outlook only works if you have the same version of Word, Excel, or PowerPoint installed as the version of Outlook being used to preview the attachment. The certificate settings for look correct. File attachments can be viewed in the Reading Pane or in the body of an email message without opening another program. I checked the IMAP settings and it all looks correct. Even if they click "Continue" it never goes through.

So quit Mail.app, open Terminal.app, paste in the above line, and re-open Mail.app. "A secure connection cannot be established with the server etc etc etc". Q: Right after I upgraded my Mac to the Big Sur 11.3 operating system, I could no longer send e-mails with attachments using the Microsoft Outlook program. To make Mail.app always use an icon for single page attachments you need to paste the following into Terminal.app: defaults write DisableInlineAttachmentViewing -bool yes. They can send a plain text based email fine, but when they try to send anything with an attachment (no matter the attachment size), they get this error (gmail account specific, for this message, but they can't send attachments from any of their accounts in Outlook): They have several different email accounts in their Outlook (v16.48 for Mac).

Working with a relative a few time-zones away on this one.
