Issue

When working in Microsoft Outlook (switching between emails, opening a new email, clicking reply, etc.) following message randomly pops up and Outlook is unusable until it goes away (normally in 2-3 seconds).

Contacting the server for information

Resolution

The issue was resolved by enabling option Don’t download pictures automatically in standard HTML email messages or RSS items in Outlook’s Trust Center. 

File > Options > Trust Center > Trust Center Settings > Automatic Downloads.

Microsoft Outlook 2016