You're welcome.
Yes, if you had 20 there, that was the problem. You can either leave the field blank or use a higher setting.
I don't have a clue how it got there.
If having entered it there by accident is not a possibility, this smells like a preferences corruption issue. If you observe other problems or strange things happening in Mail, consider re-creating the preferences file, or just do it anyway to be on the safe side:
1. Quit Mail.
2. Make a backup copy of the
~/Library/Mail folder, just in case something goes wrong. You can do this in the Finder by dragging the folder to the Desktop while holding the Option (Alt) key down, for example. This is where all your mail is stored.
3. In the Finder, go to
~/Library/Preferences/. Locate
com.apple.mail.plist and move it to the Desktop (to be deleted afterwards).
4. Open Mail. You'll have to set up your non-.Mac accounts from scratch all over again. If given the option to import existing mailboxes or something like that,
don't. Just enter the account information and Mail will automagically rediscover the data in
~/Library/Mail/ when done.
You'll also have to re-configure most of your settings in
Mail > Preferences. For spam-related security reasons, the first thing you should do is go to
Preferences > Viewing and disable
Display remote images in HTML messages if it's enabled.
5. As a side effect of re-creating
com.apple.mail.plist, Mail may have renamed
Outbox (which is where messages waiting to be sent are stored) to
Delivered. The name of that mailbox is actually a misnomer, as it would contain messages (if any) that couldn't be delivered for some reason. You can delete that mailbox if you wish.
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 file path here, doing Go > Go to Folder in the Finder, and pasting the file path there.
