It seems that many servers do not support the native php mail() function, causing problems in form submission. Would it be a good idea to integrate PHPMailer into the Sitejet applications? This would help many non-technical users ( including myself). Additionally, if I’m not mistaken, users who install PHPMailer manually must reapply changes to the api.php file after every publish, typically by restoring the previously modified version.
Thank you for posting this request. Unfortunately, this is outside of our development scope as this has to be decided and done by Plesk or cPanel. We would also be unable to realize this from our side as we have no access to their systems to do this kind of development.
I do strongly agree that this can be a lot for a non-technical user! But it is the responsibility of Plesk and cPanel as well as the providers offering their services to ensure that every user has a chance to use the tools given properly.
Hope this gives some clearance about your proposed request.
I think the best is to post this in their community. 
Thank you for the clarification.
I understand that server-side PHP mail() availability depends on the hosting environment. However, my question was not about enabling PHP mail() on the server, which itself may also raise security concerns.
My concern is that the Sitejet-generated contact form appears to rely on the native PHP mail() function in api.php. If users have to manually modify api.php to use PHPMailer or SMTP (which alone may already be too complicated for many of us), those changes are overwritten after each new publish. For that reason, this seems to be a Sitejet Builder/form-handler limitation rather than only a server configuration issue.
Of course, this is only my non-technical understanding of the situation, so I may be mistaken. I also do not mean this as an argument or complaint — I simply wanted to suggest a possible solution (for example, PHPMailer integration instead of relying on the native mail() function) to an issue that seems to come up quite frequently for users.
Could you please clarify whether native SMTP/PHPMailer support in the generated form handler is something that should be requested from cPanel/Plesk rather than Sitejet directly?