Mail crashing when sending via Gmail

Question

I've been sending and receiving through my Gmail account just fine for months now. However, over the last two days or so, Mail will get stuck in a crash loop if I attempt to send a message through Gmail - but not any of my other accounts. Mail behaves fine if I send through .Mac or any of my personal hosting email accounts.
When trying to send through Gmail, Mail will crash while appearing as though it is still trying to send the message. The kicker is that, often times, the message actually gets sent out; this has resulted in duplicates being sent to friends and a couple email lists I'm on.
Upon starting Mail back up, it attempts to send the message again - which causes another crash. The only way I've been able to stop this is to use the keyboard shortcut to quickly bring up the activity panel and cancel the sending operation before the crash occurs.
I have removed all plugins I use (MailTags and Mail Act-On) in a troubleshooting effort to find out if they were causing the problem. Nothing changed; Mail keeps crashing when sending through Gmail.
Any thoughts are much appreciated.

Answer 1

Verify/repair your disk first (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
When an application crashes (i.e. quits unexpectedly), the system writes information about the state of the application at the moment of the crash into a log file. That information is called a crash report and it makes troubleshooting much easier. In order to find and paste the relevant part of the crash report here, proceed as follows:
1. Open ~/Library/Logs/CrashReporter/Mail.crash.log. The whole Mail crash log appears on a Console window (you can also open that file with any text editor). It may contain many crash reports. We only want the latest, which is located at the bottom of the file.
2. Look for a line that begins with "Date/Time:" as close to the bottom as possible, and another that says "Thread N Crashed:", where N is a number. Select everything from the "Date/Time:" line to the lines immediately following "Thread N Crashed:". Copy that text and paste it here. Do not post the whole crash log, and do not include anything beyond the lines immediately following "Thread N Crashed:" either, just the relevant part.
Take a look at the following thread for examples of what a crash report looks like:
http://discussions.apple.com/thread.jspa?threadID=466701
In the meantime, you may try re-creating the preferences file, in case there is something wrong with it (the account settings are stored in this file):
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 the com.apple.mail.plist file and move it to the Desktop (to be deleted if this fixes the problem).
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.
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.