Problems with Mail Since OS Upgrade

Question

Since the upgrade to OS 10.6.7, I am having trouble with Mail interacting with Gmail. I can't send emails out. Port 993 times out. Occasionally, I can get emails in the Outbox to go out upon opening Mail but not thereafter. I have reconstructed the p-list file, and pared down my mail accounts to a single gmail account, but still nothing. Connection Doctor shows smtp.gmail is connected (green).
For what it's worth, my iPad running on the same network with the same ISP sends and receives email fine from all accounts.
Now what?
Thanks in advance for any suggestions.

Answer 1

The most important step is that mail is closed through out all of this:
Close mail.
Clear messages trying to send in outbox by either deleting or saving as draft
Open TextEdit
Go to File
Click on Save As
Click on the triangle to the right of where it says "Untitled" to make it point down
Click on where it has your name next to the house symbol on the left hand side
Hold Command-Shift
Hit the Period button once while your holding the above two
Should see a bunch of new stuff namely "DS_Store"
Click on the Library folder
Click on the Mail folder
Click on the folder with your email address on it
In there you should see something called .offlinecache
Click on the Trash to open a window and drag the .offlinecache into there (you cannot just drag it to the icon of Trash you have to drag it into the window)
Restart, empty trash, open up mail, should work fine