Synchronizing folders

Outlook is constantly “synchronizing folders.” What is this? How can I stop this? It’s slowing down my computer.

"Synchronizing folders" is the generic term for when Outlook is sending and receiving messages and when (if your mail account type supports it) uploads changes to the mail server. Under certain circumstances this process can "hang" and indeed can cause your computer to slow down quite a bit. Below you’ll find some common reasons for it and how to troubleshoot it.

Send/receive interval too short
Outlook can automatically check for new messages on a set interval. When you set this interval too short, a new auto check will take place before the previous one can finish and thus the send/receive tasks can accumulate so that Outlook is basically stuck in an endless synchronization process. You shouldn’t set the automatic send/receive interval to less than 10 minutes. When you have more than 3 accounts I wouldn’t recommend to set it shorter than 15 minutes. A good rule of thumb is to add about 5 minutes for every 2 extra mail accounts.
You can set the send/receive interval in;
Tools-> Options-> tab Mail Setup-> button Send/Receive…

Stuck message
When you have a message stuck in the Outbox, Outlook could end up trying to send that message until it succeeds. When it is truly stuck (for instance because it is too big), this of course will never work. In most cases you can see the message and you can use these techniques to remove it. When you don’t see the message, but Outlook still indicates it is trying to send a message at every send/receive interval, it could be a stuck Read Receipt and then you can use this method to remove it.

Virus scanner integration
Virus scanners are a known source to cause all sorts of connection issues. It also applies in this case. You should really disable it as it doesn’t offer an additional level of security anyway.

Check send/receive process
In some cases it could be that just a single mail account is hanging. This could for instance be caused because the ISP for that mail account is down. In those cases you can contact your ISP and ask what is wrong. You can sometimes also get an indication of what is going wrong when there is an error reported in the Send/Receive dialog box. When there is an error, the dialog box usually shows itself with the Error tab revealed. You can look at it manually as well via;
Tools-> Send/Receive-> Send/Receive Settings-> Show Progress…-> tab Errors