Did you consider asking your hosting exactly what SMTP settings are required to send email from your website? X3 simply forwards those details, through the industry-standard
PHPmailer. X3 (or any other web app) can't send emails for you ... It's done either from SMTP (recommended) or the server's own emailer (dodgy).
shoot_machine wrote:And SMTP is necessary at all? Asking as my still running old Koken installation works without and I actually wanted to replace it.
Did you try? Just disable SMTP option. SMTP is not strictly required, but in modern days, it's the recommended approach to avoid many issues (spam-sorted email, server blacklisting). Contact forms on your website will send email FROM you, TO you, on behalf of the person who fills the form. Therefore, the FROM (actual sender of the email) is important, and SMTP is often used.
shoot_machine wrote:Asking as my still running old Koken installation works without and I actually wanted to replace it.
X3 can of course send email on the same email options you have in your Koken, but I don't know what they are. If you are sending email without SMTP, it means your server has kept the localhost emailer enabled ... In this case, I am guessing they have some strict requirements that the FROM address must be from an email on the same domain.