Mail producing lots of network traffic in background

Hi Neil.

You probably have Preferences > Accounts > Advanced > Automatically synchronize changed mailboxes ON. If that‘s the case, turn it OFF.

In that case, there is probably something amiss somewhere.

What’s the time interval chosen in Preferences > General > Check for New Mail? If it’s set to Every minute, try changing it to Every 5 minutes.

Try rebuilding the affected mailbox (if you can determine which one it is), i.e. select it in Mail and do Mailbox > Rebuild -- it may take a while for Mail to complete.

Do you have any Mail plug-ins or system utilities that could interfere with Mail? In the Finder, go to each of the following folders (if they exist). What do you see there?

/Library/InputManagers/
~/Library/InputManagers/
/Library/Mail/Bundles/
~/Library/Mail/Bundles/

Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. You can easily locate any of the folders referred to in this post by copying the folder path here, doing Go > Go to Folder in the Finder, and pasting the folder path there.

Verify/repair the startup disk (not just permissions), as described here:

The Repair functions of Disk Utility: what's it all about?

Go to Apple Menu > System Preferences > Network, choose Network Port Configurations from the Show popup menu, and make sure that the configuration used to connect to Internet appears at the top of the list.

Finally, try this:

1. Quit Mail if it’s running.

2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is stored.

3. Locate Envelope Index and move it to the Trash. If you see an Envelope Index-journal file there, delete it as well.

4. Move any “IMAP-”, “Mac-”, or “Exchange-” account folders to the Trash. Note that you can do this with IMAP-type accounts because they store mail on the server and Mail can easily re-create them. DON’T trash any “POP-” account folders, as that would cause all mail stored there to be lost.

5. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index -- Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.

6. As a side effect of having removed the IMAP account folders, those accounts may be in an “offline” state now. Do Mailbox > Go Online to bring them back online.

Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder, i.e. ~/Library is the Library folder within the user’s home folder.

I have been experiencing this problem as well. Apple Mail constantly uses all available upstream bandwidth, and quite a bit of the downstream bandwidth too. However, rearranging the network ports did not help me.