When setting up a new user in Direct Admin, you need to be aware of the way Direct Admin creates a default email address, associated with the new User Account.
As you would know, the User Account name, is used as the default email account, and has the User Account password as it's password.
If your customer happens to want that email account as an active, usable email account, there is a slight change of tack required when setting it up in an email client, or even logging into it via webmail.
The default user account email address, only requires the account name as the username, when using Outlook or Webmail. Unlike a normal email account, which requires the full email address as the username.
I spent several hours trying to fault diagnose why my client could not access their email via that email account. Pure fluke on my part discovered the requirement of only the User Account Name as the Username. If you're a website builder, or host websites, Direct Admin is a great little Control Panel that runs very fast and snappy and uses far less resources on your webserver than CPanel or Plesk.
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