When you select for a virtual server to use the "user@domain" format for usernames under Server Templates > Default Settings > Mail for domain.... IMAP and POP3 work correctly, however under CentOS saslauthd never gets the correct "-r" flag needed for SMTP authentication to work with that format for usernames.
Additional flags to pass to saslauthd on the command line. See saslauthd(8) for the list of accepted flags. FLAGS=-r
"-r" needs to be added to the FLAGS configuration section there.