How ftp accounts for default domains on owned IP's work

Proftpd supports IP based FTP hosting.
DA makes use of this for owned IP by adding virtulahosts for owned IPs in the /etc/proftpd.vhosts.conf file.
These owned accounts use their own password file (stored by the user) in:
/usr/local/directadmin/data/users/username/ftp.passwd
All other accounts on shared IPs are all stored in /etc/proftpd.passwd.

An owned IP address can still have multiple domains on it (all setup under the 1 User account).
For the first domain, it's labeled as the "default domain". For this domain user accounts do not use the @domain.com syntax at the end of the login name. A simple "user" is all that's required. This exact format is specified on the Ftp page in DirectAdmin under the "Login" column.

If a domain isn't behaving correct (you want it to be the default domain, but it isn't), you can tell DirectAdmin that it's the default domain by editing:

/usr/local/directadmin/data/users/username/domains/domain.com.ftp

and specify:

defaultdomain=yes

instead of:

defaultdomain=no

This will tell DA to create ftp accounts for this domain without the @domain.com syntax.