I have qmail installed but cannot see "Use of short (webmaster) and full (email@example.com) POP3/IMAP mail account names is allowed" option on mail settings page.
When trying to turn this option via command line the following error occurs
# plesk bin mailserver --set-account-type both
Unable to set requested account format: Unable to use short and full mail accounts.
exit status 1
Dovecot is installed as IMAP/POP3 server.
Only Qmail + Courier-IMAP combination supports short mail account names authentication. Install Courier-IMAP via GUI or command line
# plesk sbin autoinstaller --select-release-current --install-component courier