I tried sending a message that was too large, now I have a "hazard" sign all over my inbox (a triangle inside a circle with a !). How do I get rid of it?
MacBook Pro (13-inch, Mid 2012), OS X Mountain Lion (10.8.2)
Select and open the "Sent" mailbox and the "Drafts" mailbox for the target mail server, and see if you have that big outbound message still stuck there. If so, delete it.
If you've filled your storage quota on the server, you'll need to log in and delete some stuff.
The web mail interface (if your mail ISP offers that) can be useful for cleaning up existing messages.
To try to bring the connection to the mail server back online, you can control-click on the inbox mailbox in the left navigation of Mail.app associated with the target mail server, and see if the server is listed as offline. You'll know it's offline if you're offered an option in the pop-up to take the server back online, select that. (If it's online, try toggling it offline, then back online.)
Error after trying to send a message that is too large
