I have 4 3CX systems in place and all on the same version, one of them only has Custom Email Setup and 3CX Email in the email settings drop down, all the others have Office365, Outlook.com, Gmail etc.. and this one system cannot send email.
Is the one system that is different using 3CX Standard vs. Pro or Enterprise?
Standard only supports 3CX SMTP, however if it was running 3CX since v14 with a custom SMTP it's possible it has a carry forward configuration allowing custom SMTP which isn't offered anymore.