This is regarding the default user account, e.g. 'x', for the virtual host name e.g 'x.com' which has an email account named 'x -- unlike other email users under that host name, e.g. 'joe.x' for 'email@example.com'.
I found that I had to use IMAP to get authentication to work whereas other email accounts like 'joe.x' worked with POP.
Operating system Debian Linux 8
Webmin version 1.851
Usermin version 1.720
Virtualmin version 5.99
Theme version Authentic Theme 18.49-8
Time on system Tuesday, August 1, 2017 3:23 PM
Kernel and CPU Linux 4.9.36-x86_64-linode85 on x86_64
Processor information Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz, 1 cores