Mail 4.6 not working correctly.

I'm on an iMac running OS 10.6.8. I can receive mail in all my accounts, but am repeatedly asked to confirm authenticity of my gmail account.

Also, sending emails seems to take ages once I hit send - sometimes I have to wait a minute or more to send a simple message with no attachments.

My wife also has an account on the same computer, and she has been unable to send emails from her accounts to certain addresses which I can send to.

In addition, the Mail icon on the dock is corrupted and looks very low res.

I've checked the email account settings and they are all correct. I've also run the troubleshooting program to check the connections - again, all OK.

Something's not right and I don't know how to fix it. Please help!