all cron ebuilds in portage depend on virtual/mta, but they try to send mails with the "mail" command, which is not supposed to be there. This can lead to no mail sent. shouldn't they depend on net-mail/mailx instead? Reproducible: Always Steps to Reproduce:
No, they depend on ssmtp, so you have to edit ssmtp.conf ("mailhub=mail:25" is the default - and the error message) "Command output [...]can be turned off and no mail will be sent at all (SysV can't do this either)."
At least vixie-cron uses /usr/lib/sendmail to send mail, which should be provided by virtual/mta (whether it is configured is another matter). This bug needs to name actual packages to receive attention. Please re-open if you can provide the necessary details.