I've found a number of questions about the "floating" behavior of the Mail Activity window in El Capitan. I have a different question: In all previous versions of Mail.app, processes in the Activity window could be cancelled by clicking on the "Stop" button. In El Capitan, the Stop button no longer seems to exist. So the Activity window tells me if something is stuck, but doesn't give me any way to cancel a process. Is there any way to get that functionality back?
Edited: Here's an example of what I'm talking about. This window has been stuck on "Connecting…" for more than 20 minutes. There's no way to stop it and there's no way to find out what's going on. Turning off Wi-Fi and restarting the app (even rebooting!) didn't do anything; that "Connecting…" process is just still there.
Best Answer
The Activity window doesn't offer any method for correcting problems anymore, but its status messages will alert you when something is going wrong with your mail service, and usually help you figure out what it is.
If the Activity window shows problems with one or more of your Mail accounts, you'll want to try the two additional troubleshooting aids provided by Apple.
Apple's Connection Doctor can help you diagnose problems you're having with Mail.
The Connection Doctor will confirm that you're connected to the Internet and then check each mail account to ensure you can connect to receive mail, as well as connect to send mail. The status for each account is then displayed in the Connection Doctor window. If you're unable to connect to the Internet, the Connection Doctor will offer to run Network Diagnostics to track down the cause of the problem.
Most Mail issues are likely to be account related rather than Internet connection related, however. To help troubleshoot account issues, the Connection Doctor offers both an overview for each account and a detailed log of each attempt to connect to the appropriate email server.
Running Connection Doctor
View Log Details in the Connection Doctor
You can scroll though the logs to find any errors and see a more detailed reason for any problems. The one problem with the detail display in the Connection Doctor is that the text can't be searched, at least from within the Connection Doctor window. If you have multiple accounts, scrolling through the logs can be cumbersome. You could of course copy/paste the logs to a text editor and then try to search for specific account data, but there is another option: the Mail logs themselves, which your system keeps tabs on.
While the Activity window provides a real-time look at what's occurring as you send or receive mail, the Mail logs go one step further and keep a record of each event. Since the Activity window is real-time, if you glance away or even blink, you may miss seeing a connection issue. The Mail logs, on the other hand, keep a record of the connection process that you can review at your leisure.
Enabling Mail Logs
Apple includes an AppleScript to turn Mail logging on.
Once it's turned on, the Console logs will keep track of your Mail logs until you quit the Mail application. If you want to keep Mail logging active, you'll have to re-run the script before each time you launch Mail.
To turn Mail logging on:
Viewing Mail Logs
Mail logs are written as Console messages that can be displayed in Apple's Console application. Console allows you to view the various logs your Mac keeps.
You can now use the Mail logs to find the type of problem you're having, such as passwords being rejected, connections being rejected, or servers down. Once you locate the problem, use Mail to make corrections to the Account settings, then try running the Connection Doctor again for a quick test. The most common problems are wrong account name or password, connecting to the wrong server, the wrong port number, or using the wrong form of authentication.
Use the logs to check all of the above against the information your email provider gave you to set up your email client. Finally, if you still have issues, copy the Mail logs showing the problem and ask your email provider to review them and provide assistance.
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