This might happen with other email hosts, but I've only ever gotten the call to fix a Gmail account. (And really, why would you use another email host?) Mail will choke on that big message, and prevent you from receiving any more of your Gmail. I've fixed it multiple times before, but thanks to Glenn for digging up this more consistent and reliable fix.
———- Forwarded message ———-
From: Glenn Anderson
Date: Tue, Feb 15, 2011 at 7:01 PM
Subject: recovered messages folder
To: Jonathan Marcus
From: Glenn Anderson
Date: Tue, Feb 15, 2011 at 7:01 PM
Subject: recovered messages folder
To: Jonathan Marcus
============
"Recovered Messages" mailbox in Apple Mail
(for Gmail account that chokes on large attachment)
1.Take the Gmail account offline from within Mail and delete the Recovered Messages folder.
2. Make invisible files visible
Download the widget to show the hidden files from http://www.apple.com/downloads/dashboard/developer/hiddenfiles.html
or use TinkerTool
3. Go to ~/Library/Mail/IMAP-<username>@domain.tld@imap.domain.tld/.OfflineCache
4. Delete all the data within .OfflineCache folder – do not delete the folder.
5. Close Mail application and reopen.
Alternatively, you can find that .OfflineCache folder via Terminal.
Gmail Maximum attachment size
With Gmail, you can send and receive messages up to 25 megabytes (MB) in size.
You may not be able to send larger attachments to contacts who use other email services with smaller attachment limits.
jjmarcus tech solutions 
[…] http://j2mac.com/2011/03/fixing-apple-mail-after-youve-tried-accidentally-to-send-a-giant-attachment… […]
Thanks a bunch, Dave!
Thanks so much. Instructions were very easy to follow. :-)))))
Glad to hear, Abby!
Fantastic! Thank you!
Found your website, followed your instructions. You saved my a**. Thank you!
I cant do this..this has happened bofore to me and I was able to fix it, but I really need step by step instructions to do this.. Are you able to give me more detailed instructions? like the domain part? am I supposed to put my email address?
You just need to look for the IMAP folder that has your email address on it (instead of the text in red).
i am unable to find an offline cache folder or a terminal folder for that matter. any help? i couldnt use the apple download bc i dont have icloud, and i wasnt quite sure what to do with the tinker tool.
I understand. Because the .OfflineCache folder is invisible (the “.” at the beginning of the name makes it so), you have to use some tool to see it. The Terminal application in OS X is the classic way to do this, but something like TinkerTool can be used to tell the Finder to show invisible files.