[xwiki-users] mail server error in 4.2?

Hi All! XE 3.5.1 -> sends mail via Share mail option in XWiki's top menu. XE & XEM 4.2 -> gives an error "The message could not be sent to <[email protected]>: email server error." All tests done at the same server: Ubuntu 12.x XE 3.5.1 standalone XE 4.2 standalone XEM 4.2 + MySQL clean installation. Any fast solution available? Is it worth Jira's bugreport or am I missing something? Kind regards, Dmitry