As described, the 'Test Email' button that appears under a user account will return 'Email was sent successfully', despite sendmail not being installed (and no email being sent).

[root@katello.domain.com:1 ~]# which sendmail
/usr/bin/which: no sendmail in (/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/opt/puppetlabs/bin:/opt/vmware/bin:/root/bin)

Foreman 1.16.0
Katello 3.5.0.1