I use a VPS (OpenVz) setup. The reason I suspect it’s a roundcube problem is that all other email clients work well without any problem.
Below is the output I ran System Settings -> Re-Check Configuration
Your system has 4 GB of memory, which is at or above the Virtualmin recommended minimum of 256 MB.
BIND DNS server is installed, and the system is configured to use it.
Mail server Postfix is installed and configured.
Postfix can support per-domain outgoing IP addresses, but is not currently configured to do so. This can be setup in the Postfix Mailserver module.
Apache is installed.
Webalizer is installed.
Apache is configured to host SSL websites.
MySQL is installed and running.
Logrotate is installed.
SpamAssassin and Procmail are installed and configured for use.
ClamAV is installed and assumed to be running.
Plugin AWstats reporting is installed OK.
Plugin DAV Login is installed OK.
Plugin Protected web directories is installed OK.
Using network interface eth0 for virtual IPs.
IPv6 addresses are available, using interface eth0.
Default IP address for virtual servers is xxx.xxx.xxx.xxx.
Both user and group quotas are enabled for home and email directories.
All commands needed to create and restore backups are installed.
The selected package management and update systems are installed OK.
… your system is ready for use by Virtualmin.