Mail can't remember passwords

[George Mosley]

Can someone remind me which plist to delete (or any other idea) in 
order to get Mail to remember the passwords for my email accounts? Some 
file has become corrupted and I keep checking the <Remember Password> 
box when I am prompted but the setting doesn't stay once I quit the 
Mail programme. I tried deleting the <com.apple.mail.plist> in my 
users/library/preferences but that hasn't helped.
Many thanks in advance
George