Outlook Classic "hangs" if I want to open or copy attachments

The described behavior matches known Outlook performance issues when opening or handling attachments and when Outlook appears to hang.

Based on the available information, there are two main areas to check:

1. Check for known Outlook/Windows issues that cause hangs
   • Classic Outlook has had recent bugs where certain operations (including copy-related actions and POP profiles) cause hangs or freezes. One documented issue is with Ctrl+C and other Ctrl key combinations in classic Outlook when using IME languages on specific builds (Current Channel Version 2409 and higher). While this is specifically about copying text, it shows that recent Outlook builds can introduce hang behavior.
   • The fix for that issue is included in later builds (Version 2501, Build 18429.20000 and above) across Beta, Current Channel Preview, Current Channel, and Monthly Enterprise Channel. Updating Microsoft 365 Apps to a build that includes the fix is recommended. Use the update history pages referenced in the documentation to confirm that the installed Outlook build is at or beyond the fixed version.
2. Check for environment factors that slow attachment operations
   • Opening attachments can be significantly delayed when the underlying Windows environment uses redirected folders. On Windows 7/Windows Server 2008 R2, a known issue causes opening an attachment (or any file via ShellExecuteEx from a redirected folder) to take 30 seconds or more. If the Outlook attachment temp or user profile folders are redirected, this can cause the long delay when opening attachments.
   • If folder redirection is in use, review that configuration with IT (for example, test with a profile that does not use folder redirection, or temporarily redirect to a local path) to see whether attachment opening performance improves.
3. General Outlook/Exchange Online performance checks
   • Slow response when opening messages or attachments can also be related to Exchange Online service health or local network issues. Check Microsoft 365 service health in the admin portal to confirm there are no ongoing service incidents, and verify that local network, proxy, or firewall conditions are not introducing latency.
4. Antivirus and attachment scanning
   • Although not in the core documentation, moderator guidance in similar Outlook attachment issues indicates that real-time antivirus scanning (for example, McAfee) can cause hangs, corruption, or delays when opening or previewing attachments. As a diagnostic step, temporarily disabling real-time scanning (if permitted by security policy) or configuring the antivirus to exclude Outlook’s temporary attachment folder can help determine whether security software is contributing to the delay.
5. If hangs are related to copy/keyboard operations in classic Outlook
   • If hangs are also seen when using Ctrl+C or other Ctrl combinations inside Outlook, and an IME is in use, apply one of the documented workarounds until the fixed build is installed:
     • Revert Windows to a previous version of the IME as described in Revert to a previous version of an IME (Input Method Editor).
     • Or revert Microsoft 365 Apps to a build before Version 2409 using Click-to-Run:
       1. Open Command Prompt as Administrator.
       2. Run:

                    cd %programfiles%\Common Files\Microsoft Shared\ClickToRun
                    officec2rclient.exe /update user updatetoversion=16.0.17928.20156
          

Summary of recommended steps:

1. Confirm Outlook build and update Microsoft 365 Apps to a version that includes the latest fixes (Version 2501 / Build 18429.20000 or later where applicable).
2. Check whether folder redirection is in use; if so, test without redirection to see if attachment opening speeds up.
3. Review Exchange Online service health and local network conditions.
4. Test whether temporarily disabling or reconfiguring real-time antivirus scanning improves attachment open/copy performance.
5. If IME and Ctrl-key hangs are involved, apply the IME or Outlook version workarounds until fully patched.

References:

• Classic Outlook hangs when using the Copy shortcut
• Opening an attachment in Outlook takes longer than expected
• Outlook performance is slow in the Microsoft 365 environment
• Troubleshoot attachment problems in Outlook for Windows
• "You don't have appropriate permission to perform this operation." message when I try and open an attachment in Outlook - Microsoft Q&A
• M365 Outlook Classic alters PDF files - Microsoft Q&A