Solution:
By default PHPMailer (and thus CF7, I assume) acts as a big wrapper around PHP’s built-in mail()
function. The mail
function requires that you have a working local mail server, most commonly postfix.
To work around this, configure CF7 to ask PHPMailer to use its own SMTP client, which will then send directly via whatever relay you configure (e.g. gmail), will not call mail
and thus does not require a local mail server.