If you don’t configure SMTP settings in your config, sendmail (a command line utility) is used, which often fails, for some backgrounds, see the explanations here: Sending email with or without SMTP configuration? - #6 by rasteiner
KIrby uses a wrapper around PHPMailer, but using PHPMailer does not automatically mean that you use SMTP, if you don’t configure it. So that’s probably what the support wanted to suggest.