TS2621 Sometimes when opening mail, all existing messages disappear. Help please!

Question

Sometimes when opening email all existing messages just disappear.

Answer 1

Mac HD says 2.49.72GB and 151.88 used??
That's impossible. I think you mean to say that your HD capacity is 249.72 GB, and you've used 151.88 GB. You do have enough free space, so that means something else is going on to cause this problem.
Aside from setting up the Gmail account(s), did you install or update anything just prior to this problem starting? If so, what were those items?
I didn't mention any extension, so I'm assuming you're referring to seventy one's mention of TM (Time Machine). That is a built-in backup method that you can choose to take advantage of, but I would not do it, since it's not all it's cracked up to be. What you need is to setup and maintain every day a bootable backup of your entire hard drive. You can do that with SuperDuper or Carbon Copy Cloner. You don't need an Airport device for these, nor Time Machine; in fact, I would shy away from using wireless for such things, as it's often more of a problem than a benefit.
Also, there may be problems with your hard drive, so I would boot your Mac from the install disc, choose your language, then wait about 15 seconds for the menu bar at the top to appear and choose Utilities->Disk Utility and Repair Disk. If it comes back with any errors that can't be fixed, post them in your reply. But if not, then run Repair Permissions, and all you need to concern yourself with is the message at the end that 'Permission repair completed'; ignore anything else as long as you see that message.
Then reboot your system and restart Mail and setup your account(s) again (assuming it's not there because you deleted the IMAP mail folder and the Envelope Index file. In the future, don't throw things away unless you know the consequences of doing so; you'll end up being unfortunately surprised when you lose critical information.