Hmm. I'm hoping we're not seeing two different versions of the web panel, but here goes step by step:
Go to Mail -> Addresses
This should bring up a two-part table. The top part starts off with this:
"All email addresses, showing mailboxes first then addresses by domain."
There will be three columns. The one in the middle is labeled:
"Mailbox Name" Login
Disk / Max (%)"
a. E-mail Address is the primary address for the mailbox
b. "Mailbox Name" is just a label, it doesn't do anything.
c. Login is the username on the machine.
If Login is not m9999999 format, then there should be a correspending user in the Users -> Users panel.
All users from the Users panel should appear as a "Login" in this column. If you have a user on the Users panel that is not listed as a "Login" in this column, then there is no primary address for its mailbox (must have on to be listed here). To fix this, choose "Create a new address", then check "deliver to this mailbox:" and select the missing user from the "Mailbox Login" dropdown menu.
You might see somethind different, the URL for mine is
Perl / MySQL / HTML+CSS