Virtualmin configures the Dovecot POP3 and IMAP server for usage with all common mail clients, and Cyrus saslauthd for SMTP authentication on outgoing mail.
POP3/IMAP Username
You can always find the username for a user by looking on the Edit Users page for the virtual server in which the user exists. It will be displayed in the IMAP/FTP login field. With the exception of users with @ in them, the username will be the same for all services (mail, ssh, FTP, Usermin, etc.).
Username Conventions
Virtualmin, by default, creates system login names by combining the username and the first part of the domain name, separated by a configurable separator ("." by default). Thus, a user named joe within the virtualmin.com domain would use the login name joe.virtualmin.
This is, of course, completely configurable. To choose the username format used, browse to System Settings:Server Templates and select the Mail for domain section, and locate the Format for usernames that include domain field at the bottom of the page.
user@domain.tld Style Usernames
Virtualmin also supports user@domain.tld style usernames, though there are some caveats in this configuration. Specifically, Postfix does not support delivery to users with @ in them, and so Virtualmin creates two users (one that Postfix will deliver to, and one with @).
In order for SMTP authentication to work with users in this format, saslauthd also needs to be configured to include domain information.
On Red Hat, Fedora, and CentOS systems, edit /etc/sysconfig/saslauthd and add "-r" to the FLAGS= line, save it and restart the saslauthd service.
On Debian and Ubuntu, edit /etc/defaults/saslauthd and add the "-r" flag to the PARAMS= line, save it, and restart the saslauthd service.
Thunderbird
Open the Account Settings dialog. Click the Add Account button in the lower right corner of the window. Select Email account and click Next.
Fill in your name and the email address for this account.
Select POP or IMAP, fill in the hostname of the Virtualmin server in the Incoming Server, and then click Next.
On the next page, fill in the username. Click Next.
Give the account a name (the default will be the email address associated with the account).