If mail is not configured, password reset links will not work and you will have to manually/reset passwords for everyone.
Default configuration is shown below:
## SMTP/Mailer settings
smtp['reply_to'] = "email@example.com"
smtp['from'] = "firstname.lastname@example.org"
smtp['address'] = "my.smtpserver"
smtp['domain'] = "packagecloud.io"
smtp['enable_starttls_auto'] = true
## Comment out the following lines to disable authentication for SMTP
smtp['authentication'] = "plain"
smtp['user_name'] = "my username"
smtp['password'] = "my password"