How to enter SMTP mail config without SMTP login?

my SMTP server uses IP-based auth (must connect within the subnet), and so there is no login, but i want to use my SMTP server in order to use the DKIM config.
if i set it to PHP-based mail, it sends to my domain fine, but i need it to send through my SMTP server.

so how can i configure SMTP mail, without the username:password login info? i do have some knowledge of PHP, but apparently not enough to find the code needed to change...

Thanks in advance for any help.

It would be helpful to post your OpenCart version. Assuming 4.0.x you will probably need to edit system/library/mail/smtp.php. Look for the 'AUTH LOGIN' part.

sorry, v4.0.2.3
found the code, and it looks like not having a username and password is handled, except that it generates an error if not filled in. so i will comment out the error generation, and see what that does for me... and it works! Thank you very much! i will try and be more diligent next time and post better info, like the version number.

I wrote complete tutorial on how to setup OpenCart email here:

https://hpwebdesign.io/blog/email-settings-opencart

** Both use SMTP or mail() function on PHP (make sure is not blocked by hosting)