SEB-APPSTORE,
I had a similar issue this how I dealt with it.
older version it may be creating problems. (they just did this starting 6/10)
WRITE DOWN ALL INFO IN ALL EDITABLE PANES IN CASE NEEDED LATER
I had this issue with passwords before and I first went and changes the password with the ESP, then I went to keychain access, passwords category, there you should see 2 mail entries imap. mail.xxxx..., and another with your email @ xxxx....
They should say application under kind column
Double click one one of them to open, then click show password, it will prompt you for the admin password, and probably again with user as well as password. Enter that info and it should show you the password, delete the password and re-enter to match the web based mail password, click save.
Repeat this for the other one, basically the same drill as above.
Then launch mail and sent something to yourself, like test, it may now prompt you for your password, enter it and click remember in my keychain, then click get mail it may a minute or two for the server to pass it along but it should come.
It took me a while to remember this, as I did it quite some time ago, when my esp changed the mail, previously.
Hope this helps